This happens when VirusScan can't unpack a file, often due to corruption in the file or perhaps that it is a zipped file containing other pre-zipped files. The only solution to that is to move the offending files to external or removable media.
It can also happen when there are disk read errors. Go to Start and type in the search box cmd, in the space above when cmd.exe appears right-click it and "Run as Administrator". Enter chkdsk /r (with the space) and click Enter. You'll get a rejection saying error, volume in use, try at reboot Y/N or words to that effect. Enter Y and click the Enter key. Exit the command prompt window and reboot. Let it run its course and see if that cures the problem.