Q: My iMac is overloaded with a virus. Avast couldn't intsall.

Dont install Avast, it is junk. And you don't have a virus, its probably adware

Run a report using this tool and post it here

The problems you describe are not caused by a "virus", and seeking an "anti-virus" solution is inappropriate. Do not install "Avast" or any product resembling it. To do so will only compound your miseries.

Please read How to install adware.

Follow Csound's instructions.

There are no known viruses that can attack Mac OS X. All these so-called AV apps are selling a cure for a disease that doesn't exist. Worse yet, they are renowned for borking Mac systems.

A Mac can pick up browser pop-ups and the like, but not if you just use common sense. You really have to actively invite some malware in for it to happen. That means downloading and knowingly giving your password for it to download. Mac provides all the protection you need with SIP (System Integrity Protection).

About System Integrity Protection on your Mac - Apple Support

Also stay away from programs that purport to "clean your Mac". Your Mac needs no cleaning, all that is taken care of quite well by the system.

You may have installed ad-injection malware ("adware").

Don't use any kind of "anti-virus" or "anti-malware" product on a Mac. There is never a need for it, and relying on it for protection makes you more vulnerable to attack, not less.

Back up all data first.

If you're not already running the latest version of OS X, updating or upgrading in the App Store may cause the adware to be removed automatically. If you are already running the latest version, please log out or restart the computer. Again, some kinds of malware will be removed—not all. There is no such thing as automatic removal of all possible malware, either by OS X or by third-party software. That's why you can't rely on software to protect you.

If the malware is removed in your case, you'll still need to make changes to the way you use the computer to protect yourself from further attacks. Ask if you need guidance.

If the malware is not removed automatically, see below.

This easy procedure will detect any kind of adware that I know of. Deactivating it is a separate, and even easier, procedure.

Some legitimate software is ad-supported and may display ads in its own windows or in a web browser while it's running. That's not malware and it may not show up. Also, some websites carry intrusive popup ads that may be mistaken for adware.

If none of your web browsers is working well enough to carry out these instructions, restart the computer in safe mode. The malware will be disabled temporarily.

Step 1

Please triple-click the line below on this page to select it, then copy the text to the Clipboard by pressing the key combination command-C:

~/Library/LaunchAgents

In the Finder, select

          Go ▹ Go to Folder...

from the menu bar and paste into the box that opens by pressing command-V. Press return. Either a folder named "LaunchAgents" will open, or you'll get a notice that the folder can't be found. If the folder isn't found, go to the next step.

If the folder does open, press the key combination command-2 to select list view, if it's not already selected. Please don't skip this step.

There should be a column in the Finder window headed Date Modified. Click that heading twice to sort the contents by date with the newest at the top. If necessary, enlarge the window so that all of the contents are showing.

Follow the instructions in this support article under the heading "Take a screenshot of a window." An image file with a name beginning in "Screen Shot" should be saved to the Desktop. Open the screenshot and make sure it's readable. If not, capture a smaller part of the screen showing only what needs to be shown.

Start a reply to this message. Drag the image file into the editing window to upload it. You can also include text in the reply.

Leave the folder open for now.

Step 2

Do as in Step 1 with this line:

/Library/LaunchAgents

The folder that may open will have the same name, but is not the same, as the one in Step 1. As in that step, the folder may not exist.

Step 3

Repeat with this line:

/Library/LaunchDaemons

This time the folder will be named "LaunchDaemons."

Step 4

Open the Safari preferences window and select the Extensions tab. If any extensions are listed, post a screenshot. If there are no extensions, or if you can't launch Safari, skip this step.

Step 5

If you use the Firefox or Chrome browser, open its extension list and do as in Step 4.

This site has had ongoing problems with posted links. This one should work: How to install adware