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First try updating your VirusScan, a new DAT that covers that malware might be available. If that gives no joy then try scanning your PC in safe mode:
Tap F8 repeatedly while booting up. You'll get a boot screen with choices.
Pick Safe Mode, or Safe Mode with Networking if you will need access to the
internet. Your PC will boot in a lower resolution state as most processes wont be running.
Go to "My Computer" (XP) or "Computer" (Vista/Windows 7), right-click the hard drive
and select "Scan" from the drop-down menu. All you'll see is an extra taskbar icon which will show a progress report if you hover over it.
If still no good scan in DOS using McAfee:
If things are still unstable or your want a second opinion then to be on the
safe side scan with the free versions of these tools:
Sometimes the malware could be protecting itself against this by preventing
the installation so try this:
Rename the executable..
Example: open the C:\Program Files\Malwarebytes folder, then rename the
"mbam.exe" file to something else, like "1234.exe", then double click directly on the file to open the program.
Update both programs before running them and let them clean anything they find by doing a full scan.
If they quarantine a file or cannot remove a file try to get a copy of it and send it to McAfee. Guidelines published HERE.