first run your virus scan (or avast) at boot time or safe mode using deep-scan modes. (boot-time is important.)
then download and use the latest lovesan/blaster/sasser removal/fixer tool/s from microsoft/symantec.
take all windows updates. but ensure that u have patched up your pc with atleast KB835732 patch.
download and use the latest microsofts malicious software removal tool. also install sp2 if u havent, and enable its firewall.
else use zone alarm firewall, when on the net.