I would avoid Windows Vista like the plague ... Windows 7 RC seems a bit better, but I'd stick with XP if you are comfortable switching over to Linux. (As a matter of fact, I would also avoid Macs as well, simply because I find them to be incredibly overpriced compared to PCs - remember not to get off topic and start a Mac vs. PC war guys

But as far as the Linux goes, depending on what distro you use, setting up a media player to watch anime can be a pain in the ass, or easy as hell. If you take something like Linux Mint, you can just throw your video file at Totem, or Mplayer right after the installation.
I'd also recommend looking into XBMC for Windows, Linux, or Mac - it kicks total ass when you have a collection of video files.