Witness Security Essentials, a new anti-virus utility that's fast, effective, and free. When combined with the spyware-fighting Windows Defender (which is now baked into the OS) and the Windows Firewall (ditto), it provides nearly all the security most PC users need.
You don't have to take my word for it: Windows SuperSite's Paul Thurrott calls Security Essentials "a near perfect security solution."
Meanwhile, check out PC World's review, which concludes: "Microsoft Security Essentials got it exactly right."
What pleases me most about MSE is its small footprint: It doesn't noticeably impact system performance. And, like Windows Defender, it goes about its business quietly, in the background, making its presence known only if there's a problem.
MSE works with Windows 7, Windows Vista, and Windows XP with SP2 or SP3. Amazingly, it's free, meaning you now have the opportunity to cut the cord from subscription-based anti-virus offerings from McAfee, Symantec, and the like. That could put an extra $50-60 per year (per PC!) in your pocket.
If you've tried Microsoft Security Essentials yourself (or don't plan to for some reason), share your thoughts in the comments.
In the meantime, here's one other tool I recommend throwing into your security mix: Web of Trust, a free browser add-on that protects you from unsafe links.