What to do when a website says you have malicious software on your Mac? If you are browsing a website and it claims to have detected malicious software, malware, or a virus (or simply states it has "detected problems" with your Mac), then follow these steps:
If you're in Safari (or another web browser) and get a warning about malicious software, virus, or problems detected, quit Safari by choosing Safari > Quit. If Safari refuses to quit, force-quit the app (Control-click the Safari icon and choose Force Quit).
Go to the Downloads folder and drag any install files, or files that you don't recognise, to the Trash.
Empty the Trash (Control-Click the Trash icon and choose Empty Trash). By doing this you will dodge the problem in the first place. You haven't installed any software, or handed over your credit-card details. We'd suggest you don't return to that website.
That's how to avoid falling into the trap of installing malware to remove malware, but how do you remove malware...
Here's what to do if you suspect you've already installed malware on to your Mac - typically because a website claimed to have detected problems. Follow these steps to remove the software:
If the app is open, check the name of the app.
Move or close the app window.
Open the Utilities folder (Command-Shift-U).
Open Activity Monitor.
Choose All Processes.
Go to the Utilities folder in the Applications folder and launch Activity Monitor.
Look for an app called MacDefender, MacSecurity or MacProtector. Or look for the name of the app (from Step 1).
Click the Quit Process button (top-left) and select Quit.
Quit Activity Monitor.
Open the Applications folder and locate the app (called MacDefender, MacSecurity, MacProtector, or some other name).
Drag the unwanted app to the Trash. Empty the trash.