I had a PC with the MSN Messenger virus to fix. The virus program supplied with the PC had expired and not been renewed. This post describes some of the steps which may be useful to others in removing a virus.
So de-installed the existing virus program and installed Avira AntiVir ( http://www.free-av.com/ ) a free antivirus program which deletes as opposed to quarantines viruses. If found and destroyed the virus, but on startup 2 dlls were trying to find their code.
Eventually found that they were in the registry so used regedt to remove (be very careful here). JJ pointed me to a utility that could do this AutoRuns (below).
Then I de-installed AntiVir and downloaded and ran AVG ( http://free.grisoft.com/ )
Another set of viruses were detected and removed. Last I heard the machine was still running (whew!!), have left AVG on.
Another program you can try to remove common viruses is a utility from McAfee called Stinger ( http://vil.nai.com/VIL/stinger/ ), which you can try before going through the steps above.
But, as always... don't let your virus software one run out. If you want either renew your subscription, or replace with AVG or AntiVir
This utility, which has the most comprehensive knowledge of auto-starting locations of any startup monitor, shows you what programs are configured to run during system bootup or login, and shows you the entries in the order Windows processes them