Last August, I was at my nephews place, and their PC was infected with some kind of virus/spyware. They had an antivirus installed, a firewall + an antispyware, all were unable to get rid of it. As soon as I killed the virus task, another one started and rewrote a different entry in the registry to relaunch itself after reboot.
I used autoruns / process explorer from sysinternals but it didn't work.
Finally, I removed their AV+Antispyware (cannot remember which brand it was) and I installed CA Internet Security Suite 2007. It stopped the active virus task immediately after installation, and after reboot, it was gone, after removing/quarantining a large number of infected files, fortunately all non essential to the system (it was probably more a spyware than a virus). I know CA AV is generally ill-considered by many observers/reviewers. This time it really helped.