The one market Microsoft never went into was Spyware/Adware protection. Then came AntiSpyware/Defender. It was a huge step, and a good one.
Recently, Windows OneCare was released. This is an antivirus. It does not only malicious scans, but performance/defrag/registry things.
Windows Vista keeps sounding better and better. It really is revolutionary (arguably). This antivirus should have come with XP =)
Try the free onlien scan yourself. I did, and find some nice things (nothing malicious
)