If you are using WinXP, and to some extent with (some) other OS, the automatic backup/recovery "features" can cause persistent reinfection with a virus that manages to get on your machine.
Since the autorecovery system makes frequent copies of the registry, and will restore the "last best copy" if something changes unexpectedly, a virus that uses a changed registry entry may be "relaunched" if autorecovery restores to a backup registry copy that contains the virus the next time you restart the machine. This can be a perpetual cycle.
There have been numerous virus versions and variations that attempt, with varied success, to disable AV programs. There have also been numerous attempts to launch DDoS attacks on major AV providers' websites to prevent people from getting recovery tools.
The general procedure for such infections, or when one is suspected, is to:
1. Turn OFF autorecovery. This will delete all previous registry backups.
2. Go to a known reliable AV provider website, and use the "online scan for infections" available. This will use current AV signatures, and usually can do a valid scan.
3. Check the AV provider website(s) for any identifying filenames, or registry entries that may be associated with the suspected infection, and look for a complete procedure for removal.
4. PRINT and FOLLOW COMPLETELY AND EXACTLY ALL STEPS in any removal instructions you find.
In most cases, step 2 will actually accomplish disabling of the "virus" portion - IF YOU DID STEP 1 FIRST. Many infections carry "payload" files in addition to the "viral component" that gets the crap onto your machine, so the additional steps are needed to get rid of the "non-viral payload" junk.
In severe cases, after you have cleaned the crap off your machine, reinstallation of your AV program may be required. Once you have the system "clean" and AV working, you probably will want to turn autorecovery back on, since it is helpful in cases not associated with a virus of this kind.