Apple's newest Quicktime 7, finally released today for Windows, allows you to play high definition video files (using the video code H.264) that look simply breathtaking! If you're so inclined I recommend installing iTunes 5, which includes Quicktime 7, and then taking a look at some of these (very large files):

• Serenity movie trailer
• The Macaulay Library preview
• BBC footage from Africa
• even more...

Apple released a beta of QT7 a few months ago and the playback quality was very choppy - even on our high-powered home PC the speed was very poor, but the final release is extremely smooth, I was able to play the space shuttle launch movie on my PC at work, and it has a relatively "slow" 1.8GHz Pentium 4.