How to stop downloads that pop up every time I turn on my Mac?

It sounds to me like your machine is automatically opening Safari at startup, and Safari is automatically loading a page that triggers a MacKeeper download. If that sounds about right, here's how to make it stop. First, when you shut down or restart, uncheck the "Reopen windows when logging back in" box, as shown here:

This will prevent Safari from opening automatically. You can always check that box again the next time, if you prefer that behavior.

Next, hold down the shift key while opening Safari manually. This will prevent Safari from loading the pages that were open when it last quit. Then, if you would prefer Safari to open to a "blank slate" each time, choose Preferences from the Safari menu, click the General icon in the preferences window toolbar and set the "Safari opens with" item to "A new window."

Not only that but it warns me if I'm about to go to a unsafe website.

You can easily get that feature by downloading an add on for your browser called WOT (web of trust). It works well and does not mess up your Mac.

Please do a search here for Mackeeper and read the threads. It would be best to uninstall it.

https://discussions.apple.com/docs/DOC-3027

... If I do not keep Mackeeper how would I know what is junk and what is important for the running of a mac?

Unless you limit all your software acquisitions strictly to the Mac App Store, it's not always easy to determine what is junk and what is not (and even the App Store has its share of worthless time- or money-wasting apps). You just have to bear in mind that the Internet is still very much like the wild west, and to use your own common sense when using it or any computer on the Internet.

A Mac takes care of itself and you don't need "cleaning" software. It already includes everything it requires to protect itself from viruses and malware, as well as abilities to locate or remotely lock or erase it, should it be lost or stolen. MacKeeper can do nothing that you cannot already do more safely and effectively, using the well-engineered features already included with your Mac. MacKeeper is not the worst such utility, it is only the most notorious due to intrusive marketing tactics that are as aggressive as they are effective. Many similar products are just as bad or worse.

Keep your Mac updated with software updates from Apple. If it should ever do something odd or unexpected, ask for help from Apple or on this support website first. Following these simple principles are a far better idea than delegating your Mac's security to some product that claims the ability to do that for you.

• Never install any product that claims to "speed up", "clean up", "optimize", or "accelerate" your Mac. Without exception, they will do the opposite.
• Never install pirated or "cracked" software, software obtained from dubious websites, or other questionable sources. Illegally obtained software is almost certain to contain malware.
• Don’t supply your password in response to a popup window requesting it, unless you know what it is and the reason your credentials are required.
• Don’t open email attachments from email addresses that you do not recognize, or click links contained in an email:
  • Most of these are scams that direct you to fraudulent sites that attempt to convince you to disclose personal information.
  • Such "phishing" attempts are the 21st century equivalent of a social exploit that has existed since the dawn of civilization. Don’t fall for it.
  • Apple will never ask you to reveal personal information in an email. If you receive an unexpected email from Apple saying your account will be closed unless you take immediate action, just ignore it. If your iTunes or App Store account becomes disabled for valid reasons, you will know when you try to buy something or log in to this support site, and are unable to.
• Don’t install browser extensions unless you understand their purpose. Go to the Safari menu > Preferences > Extensions. If you see any extensions that you do not recognize or understand, simply click the Uninstall button and they will be gone.
• Don’t install Java unless you are certain that you need it:
  • Java, a non-Apple product, is a potential vector for malware. If you are required to use Java, be mindful of that possibility.
  • Disable Java in Safari > Preferences > Security.
  • Despite its name JavaScript is unrelated to Java. No malware can infect your Mac through JavaScript. It’s OK to leave it enabled.
• Block browser popups: Safari menu > Preferences > Security > and check "Block popup windows":
  • Popup windows are useful and required for some websites, but popups have devolved to become a common means to deliver targeted advertising that you probably do not want.
  • Popups themselves cannot infect your Mac, but many contain resource-hungry code that will slow down Internet browsing.
  • If you ever see a popup indicating it detected registry errors, that your Mac is infected with some ick, or that you won some prize, it is 100% fraudulent. Ignore it.
• Ignore hyperventilating popular media outlets that thrive by promoting fear and discord with entertainment products arrogantly presented as "news". Learn what real threats actually exist and how to arm yourself against them:
  • The most serious threat to your data security is phishing. To date, most of these attempts have been pathetic and are easily recognized, but that is likely to change in the future as criminals become more clever.
  • OS X viruses do not exist, but intentionally malicious or poorly written code, created by either nefarious or inept individuals, is nothing new.
  • Never install something without first knowing what it is, what it does, how it works, and how to get rid of it when you don’t want it any more.
  • If you elect to use "anti-virus" software, familiarize yourself with its limitations and potential to cause adverse effects, and apply the principle immediately preceding this one.
  • Most such utilities will only slow down and destabilize your Mac while they look for viruses that do not exist, conveying no benefit whatsoever - other than to make you "feel good" about security, when you should actually be exercising sound judgment, derived from accurate knowledge, based on verifiable facts.
• Do install updates from Apple as they become available. No one knows more about Macs and how to protect them than the company that builds them.

Summary: Use common sense and caution when you use your Mac, just like you would in any social context. There is no product, utility, or magic talisman that can protect you from all the evils of mankind.

jacval60 wrote:

... it starts downloading on my Mac even though I already have Mackeeper on my Mac.

It is unclear whether you mean you already installed MacKeeper, or whether it is merely being dowloaded repeatedly.

Merely visiting certain websites will cause a popup ad to appear that will begin downloading MacKeeper immediately, with no intervention from you. To mitigate that threat, disable popup windows in your browser. In Safari, go to Safari's Preferences, then select Security and select the box "Block pop-up windows". Some of those websites themselves will contain code that will also cause MacKeeper or other junkware to download the moment you visit those websites. The only way to avoid that annoyance is to avoid those websites. Do not reward their decisions to host such junk by patronizing their advertisers or using the services they provide.

The downloaded file itself can do nothing unless you open it and subsequently install whatever junk it contains. Never install anything on your Mac unless you are certain that you want it installed, and know how to get rid of it should you not want it.

Another reason this may occur is the presence of a "cookie" that will repeatedly cause MacKeeper advertisements to appear. You can eliminate that annoyance by removing cookies: Safari menu > Preferences > Security > Remove All Website Data. You will need to "sign in" again to any websites that require you to log in.

Visiting certain websites that have contracted with MacKeeper's marketing firms will cause those cookies to reappear, so if this continues to annoy you, repeat this procedure, and just make it a point to not frequent those websites.

MacKeeper's marketing strategies are as aggressive as they are effective. Do not fall prey to it, or similar firms that advertise "cleaning" or "anti-virus" junk regardless of whether they are "free" or not. They will cause nothing but misery.

If you actually installed MacKeeper, you must uninstall it completely. Please write back if you require instructions.

If I do not keep Mackeeper how would I know what is junk and what is important for the running of a mac?

Well, figuring out what software is junk is a good trick. Since you specify "what is important for the running of a Mac," though, we can tell you that everything is junk in that regard! Macs do not need regular maintenance, so tools that advertise their ability to "maintain," "optimize" or "clean" your Mac are full of baloney. See The myth of the dirty Mac.

Beyond that, just be careful what you download and from where. Never download from sites like Download.com or Softonic, for example, as they are known to insert adware into installers. Try to find user reviews of a program before downloading it, try it before you buy it, and always make sure that if you install something, you know how to uninstall it. (Note that generalized uninstallers, like AppZapper and AppCleaner, won't do the job properly. If an app uses an installer, uninstalling it properly will require an uninstaller provided by the developer of that app.)

What I actually like about Mackeeper is the fact if my Mac was ever Stolen, I can retrieve it. They put a tracer on it.

But that's a feature built into modern Macs for free! Turn on Find My Mac in the iCloud settings of System Preferences. See:

http://www.macworld.com/article/2034795/how-to-track-a-lost-computer-with-find-m y-mac.html

And it has virus protection also.

See my Mac Malware Guide for information about how to protect yourself against malware.

You don't need MacKeeper on your machine.
I have owned Macs since 1997 and have never put that stuff on any of my Macs and quess what they all run just like the day I bought them.

Remove MacKeeper, and don't put it back again.

If you insist on keeping and using it, contact its developer for info.