While you mention Windows Defender, have you tried Microsoft Security Essentials? which basically replaces Defender.
Yes, I have. I like it as an AV, but I like Panda Cloud better. Right now I use Win7's built-in firewall, Windows Defender, Panda Cloud Antivirus, and UAC set to its default security level. I know it's not the most secure setup, but it's light on resources and boot times. I'm careful about where I surf and what I download (and use Firefox).