Hi there,
mostlikely 1.) the Registry has become corrupt or 2.) there is a virus/worm on your system which prevents the AV progie from running...
point 1.) deinstall MacAffee with system Restore turned off, and in Safety Mode, then run a Registry fix progie, like RegHeal, RegCleaner (now JV16 Tools), or NTRegOpt to fix the Registry of your OS...
Point 2.) go to an online Virus scan site, Like that from Trend Micro, or another... have it scan your system for nasties that may have gotten into your system...
Good Luck...
Ben
If it works don't fix it! If it doesn't use a sledgehammer...