I would advise you to do a fresh install. First, backup important files (you should always do this before reinstalling your OS), and uninstall all your programs. Then, put in the installation CD for XP Media Center 2005. Restart your computer, and set your BIOS so that your computer boots from your CD-ROM drive before the hard drive. Your computer will boot into the Setup program.
At one point, it will ask you which partition to install Windows on. Choose the existing partition, and it will install right over the old copy. This will also preserve your files. However, you will have to reinstall all your programs.
If you need any more help, let me know specifically what you're having trouble with.