Once there was One Care Live or Live One Care which was free for so long that I don't think I ever had to pay for it. It was a bit slow but did seem to catch some nasty stuff that other anti-virus applications had missed. Then there was the Malware protection that came with IE8, free again, and a suggestion that the OneCare pack would be available free under a name I did publish ages ago but can't now remember. Then came Windows 7 and you may find that you need to install some new anti-virus and malware protection. Windows 7 does come with some pretty smart firewall protection and IE8 will bring the same as it had before but for peace of mind you may wish to download Microsoft Security Essentials. (Here's what they say about it in their own words. Hope they don't mind the copy and paste but saves me time!)
Microsoft Security Essentials provides real-time protection for your home PC that guards against viruses, spyware, and other malicious software.
Microsoft Security Essentials is a free* download from Microsoft that is simple to install, easy to use, and always kept up to date so you can be assured your PC is protected by the latest technology. It’s easy to tell if your PC is secure — when you’re green, you’re good. It’s that simple.
Microsoft Security Essentials runs quietly and efficiently in the background so that you are free to use your Windows-based PC the way you want—without interruptions or long computer wait times.
*as long as you're running genuine XP, Vista or 7