Windows Media Center 2005 is built on Windows XP Professional. XP Pro was stripped down to some sorts and the GUI (Graphical User Interface) was changed. It's a good OS for a Media Center PC. Mostly for MCPC's that are using a 32" - 50"+ Plasma or LCD TV's (Mostly due to the change in GUI). But, if you're only going to use it with a CRT or LCD monitor, i'd recommend XP home or PRO.
If you can't build a PC and that is what you really want, go for it. We'll see you here in a few months in the troubleshooting forum asking questions on how to fix the problem your having with your new PC.