Hmm, porn and warez sites are notorious for drive-by installations of nasties. If you go to those kind of sites (and I'm neither implying nor criticising!) I suggest you switch from IE to Firefox, which is not only a much nicer browsing experience, but also stops scripts running by default, which should help.
What other anti-malware are you running? I'd suggest Windows Defender, Spybot, Adaware and A Squared as well as an antivirus package (I like AVG) and a firewall). Between them and a little caution, your PC should remain pretty clean.