Have you emptied your internet cache files, history and your temp files lately? These can take up a lot of space.
I know you have anti virus, anti trojan and firewall setup, have these been kept up to date? I ask because there have been a few viruses doing the rounds lately (P2P networks in particular) which can disable all the above utilities. Go to Nortons site and do an online check, if nothing else, this would rule out a virus attack.