You can get a trial version of 64-bit Windows XP off the Microsoft website.
Right now, Vista requires high-spec machines, say, a video card with at least 64MB of dedicated memory, 512MB RAM, and a 1.5GHz K8 or equivalent CPU. At least, i think that's what i read.
So you might wanna go for 64-bit XP instead. Vista's gonna have a stripped-down appearance though, last i heard, so...well, it's up to you to balance the stuff out.
There are more and more 64-bit compatible apps coming out too. I think there's quite a few 64-bit AV programs available right now.
