AVG is another good free anti-virus. It also does spy-ware ad ad-ware.
Windows Defender FTW!
windows defender is nice but it's not an antivirus and not the best antispyware software. It's good to use as a second antispyware and has some nice features.
I love bitdefender and like the Avast antivirus. I've personally used both, only switched from bitdefender because I got Kaspersky for like $5