Do you have Flash running or Java enabled in the browser? Sometimes you can get a driveby spyware/malware installation that way. That's why I have Java disabled by default, Flashblock installed, and Adblock Pro installed.
As for spyware/malware software, Malwarebytes (MBAM) that was already mention is solid, as is SuperAntiSpyware. There are also some good AVs you can install now that Defender is on the way to no longer being supported. If you're super paranoid, you can boot with ClamAV, Avast, Kaspersky, F-Secure, AVG, and so on live discs.