Got a virus not found by NAV

I really need help with this one. I seem to have almost all my users infected with a virus but no one seems to know what it is. A file named avupdchk.exe is running on all of the infected computers and it disables NAV, modifies the host file so you can't get to any anti-virus websites (nai.com, symantec.com, etc.), and does not allow you to cut and paste. I called Symantec and they have no ideas. I tried cleaning it out by killing the process, applying all of the W2K patches, deleting all traces from the registry, and changing the host file back, but no luck. It continues to re-appear. Any help or advice would be greatly appreciated. Thanks.