Bruce Chambers posts this frequently in response to that question...
The type of behavior you describe, which also often applies to
Taskmgr.exe and Regedit.exe, is typical behavior of more than one
virus/worm, the three below being the most common:
Because many of the newer viruses and worms, such as the
Spybot mentioned above, can disable antivirus applications whose
definitions aren't kept up-to-date, try using one or more of the free
on-line scanners to double-check your system.
