I think its the XP. The Media Center Edition OS was never super ready for prime time.

You should try installing XP Pro and see if you still have issues. If you really truly require and need media center capabilites, try freeware http://xbmc.org/.

Also, I have been using Windows 7 RC1 for a long time now and never had any serious issues. You should try it as well, as its currently free until its expiry date (which is in over a year, which should be enough time for you to have saved the monies to buy the by then retail version). It has top notch media center capabiliteies integrated which sadly I have never used or cared about.