Microsoft now offers a free antivirus solution, called Microsoft Security Essentials. Haven't heard anything bad about it, and it seems it doesn't annoy the user like their previous paid product did.

No real personal recommendations though, my home Windows 7 install is used only as a Wintendo, with no internet browsing done, so I don't feel I need software slowing it down. If something does find it's way in past the firewall somehow, I'll just nuke and reinstall, something that Windows finds a way to make me do anyhow on a regular basis.