First, never use any kind of "anti-virus" or "anti-malware" software on a Mac. That's how you create problems, not how you solve them.
You may have installed a fake "utility" called "Advanced Mac Cleaner." Like any software that purports to automatically "clean up" or "speed up" a Mac, it's a scam, and some variants of it are ad-injection malware.
To remove it, please take the steps below. Some of the files listed may be absent in your case. Back up all data before proceeding.
Malware is always changing to get around the defenses against it. This procedure works as of now, as far as I know. It may not work in the future. Anyone finding this comment a few days or more after it was posted should look for a more recent discussion, or start a new one.
If you paid for the software with a credit card, consider reporting the charge to the bank as fraudulent.
Triple-click anywhere in the line below on this page to select it:
Right-click or control-click the highlighted line and select
Services ▹ Open
from the contextual menu.* A folder named "LaunchAgents" may open. If it does, look inside it for files with a name that begins in either of these ways:
Move any such file to the Trash.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.
Open this folder as in Step 1:
Inside it there may be one or more files with a name beginning in
If so, move those files, and only those, to the Trash. You may be prompted for your administrator login password.
Don't delete the LaunchAgents folder or anything else inside it.
Log out or restart the computer.
Open the Applications folder and move an item named "Advanced Mac Cleaner" (if it's present) to the Trash. Also remove any items with a name such as "Shoppy" or "WebShoppy." Empty the Trash.
In the Extensions tab of the Safari preferences window, uninstall an extension named "XSearch" if it's present. Also remove any other extensions that you don't know you need. If in doubt, remove all of them. None is required for normal operation. You may have to do the equivalent in the Chrome and Firefox browsers, if you use either of those.
The malware is now permanently inactivated, provided that you don't reinstall it. This step is optional.
Delete the following items, if they exist, as in Step 1:
There's no need to log out or restart after taking this step.
The problem may have started when you downloaded something, such as the application "FileZilla," from the "Sourceforge" website or some other Internet cesspit. Never visit that site again. All software should be downloaded only from the developer's website or from the Mac App Store, if applicable.
Links have been posted in this thread to the "macupdate" website. Do not follow the links, and never download anything from that site. It intentionally distributes OS X malware by packaging some free applications (such as "Firefox" and "Skype") in an unnecessary and malicious "installer."
All software should be downloaded directly from the developer's website or from the App Store. Don't trust any site such as "macupdate" that aggregates links.