eTrust EZ Armor Security Suite: now free for 1 year.
eTrust is the Microsoft corporate standard for antivirus software: it's fast, it's effective, and best of all it won't do anything funny to your copy of Windows.
Once you've installed that, don't forget to visit Protect Your PC, Microsoft's site that helps you make sure your PC is protected against Internet threats. There's even a new, handy tool - the Windows Security Advisor tool, which makes the process easier than ever before!