I second/third VLC Player. It's awesome and plays everything I throw at it. When I play .avi on Winamp, however, it becomes choppy (at least compared to VLC). At first I thought it was my codec package (K-lite) but Windows Media Player handles them fine, so I just stopped using Winamp to play video. Winamp is the best audio player for Windows though, hands down.
General commentary on Vista: My laptop (Lenovo 3000 V200) came equipped with Vista Business. The first thing I noticed was speed. Yes, the interface upgrade is nice, but in exchange for major slowdown is really not worth it. I've turned off a lot of the Aero features (transparency, maximize/minimize animation, fading windows) just to get the speed to be somewhat comparable to what I would be getting on XP with the same hardware. I've noticed that after (re)booting, it takes a little while for Vista to get up to a decent speed (due to Vista's memory management, I'd imagine). Also, shutting down takes forever, which is frustrating for dual-booters like me, who want to be able to switch operating systems rapidly. Overall, Microsoft could've done better.
Currently, I dual-boot my machine with Linux (Ubuntu Gutsy distro), and it's quite frankly impossible to return to Vista after getting used to Linux's lightning fast speeds. The only thing that keeps me attached to Windoze is that there are some compatibility issues with my sound and graphics cards and Ubuntu drivers. Anyone bragging about the pretty animation on Vista (or Mac OS X Leopard) should check out compiz fusion on Linux. Totally blows both of them out of the water. Unfortunately, there's little support from commercial software for operating systems other than Windows and Mac, which is a huge handicap.
Okay, I'm done.