There once was a pretty bad-ass virus/worm named "Conficker".
The problem is that some virus (at least Conficker did, I think) attach themselves to some windows service routine like svchost.exe.

One test if its a type of Conficker was to try to browse to any anti-virus software website.
If you cannot access them your system may be infected with Conficker, because since a certain version Conficker blocked access to thoses websites.

Quick online check if your system is possibly infected with Conficker:
http://four.cs.uni-bonn.de/fileadmin/user_upload/werner/cfdetector/

Some more information about Conficker (and tools how to remove it):
http://four.cs.uni-bonn.de/wg/cs/applications/containing-conficker/