It is free for those that have legit copies of Vista or Windows 7. Not sure about Windows Xp.

It doesn't have a firewall so use the Windows built in firewall or download a freebie like Comodo or PC Tools.

http://www.microsoft.com/security_essentials/

It is my understanding that it isn't just a re-badged version of, the now discontinued, Microsoft One Care. That was a wee bit bloated. This seems pretty lightweight so far.

Works fine on XP sp3

BTW, it cracked open some spam filtered emails and trashed them as being virus ridden. These files existed when I had One Care on the machine. I'd say it has a new detection engine.

Still looking for a lightweight firewall. I would just like a better interface than the built in firewall has.