1) What's your OS?
Did you manually remove all files and registry keys related to the virus?
If you remove components of the virus in normal mode, anything in memory can re-infect the system. 9X systems: Safe mode or booting to DOS (not restarting in DOS mode) allows you to manually clean the system without re-infection, provided you do a thorough job.
NT based systems are trickier without the ability to boot to DOS and read the file system, your only hope is safe mode, and re-infection is still possible.
2) Do you have system recovery enabled?
System recovery can keep a copy of a virus in each image. I've seen systems with 100s of copies of the same virus, most of them in the system recovery folder. Turn off SR, then clean it manually.
Viruses today often have the ability to attack antivirus and firewall programs. Loading NAV AFTER infection isn't going to get the job done, you have to do it the hard way.