Avira's pretty good, and has a better detection rate than AVG from what I've read elsewhere, but the free version has an annoying (and huge) advertising popup which appears every time the virus definitions update (easily disabled with a bit of googling).
Don't turn UAC off.
I'm not convinced Windows Defender does anything useful, but it doesn't seem to use much in the way of resources, so it wouldn't hurt to leave it running I guess.
Anything else to do with Vista, the consensus is generally "leave it alone" unless you have a very specific reason for modifying its default behaviour.