The VLC media player (http://www.videolan.org/vlc/) is a great open-source video/audio player that works on just about every platform out there. Aside from being free and multi-platform, it provides the widest support of the many video and audio formats that are out there just about the only format it does not support is the Real format since that is proprietary. You can check out the full list of supported media formats here: http://www.videolan.org/vlc/features.html.

I did have an issue getting the VLC media player to work on Vista though, the video just wasn't outputting correctly. In my case I found that the VLC video output could be fixed by going to Settings -> Preferences in VLC. Once there, expand the Video section, and then click on Output Modules. Then click on "Advanced options" in the lowe right corner of the screen. I found that by then changing the "Video output module" setting to "Windows GDI Video Output", VLC started to correctly display video for me in Vista.