Why do videos run slow on my computer?
Answer: First of all, make sure you have the latest driver software for your video card. To get the latest version, you can go to the video card company's web site and download the driver update for free. I would suggest defragmenting the hard drive, since excessive hard disk scanning can prevent smooth audio and video playback.
If neither of these options do the trick, try upgrading to the latest version of the software you are using to play the movies. Finally, if that doesn't work, you might just not have enough hardware power to play the video smoothly. That means you need to actually go buy a new video card if you want smooth, high-quality video. Doh.