i'll give hijackthis a heary second as well.
i use that, system suite 7 antivirus and firewall, adaware and spybot search and destroy.
spybot S&D now had a TSR in it that is very very good
grab a good firewal as well, as was mentioned, windows firewall is ok, zonealarm is pretty good as well, especially the paid versions.
there is also a anti-rootkit software called "blacklight" if i remember correctly. damn good, but it's a paid software after trial.
I used it about a year or so ago to clean a friends computer.