Microsoft are going to be releasing Security Essentials later today, for those that don't know this is their free anti-virus, anti-spyware, well anti-malware in general application.
I've been running it for a few months now on all my systems, and the short review is this: it's the best anti-malware software ever, it doesn't slow your system down noticibly and it isn't constantly bugging you with pop-ups like other anti-malware software.
My recommendation is this: If you're using Norton or McAfee, stop giving them money, uninstall their terrible products and replace it with this. Check out my other coverage of it here.
Keep an eye out on the Security Essentials website for the download links.
Update: it's now up.