Vista Ultimate Upgrade Lockup

Im trying to upgrade my XP Media Center PC to Vista Ultimate. Ive run the checks to make sure everything is compatible and it all checks out. The problem Im having, is that when I attempt to install, either upgrade or just complete install, the Vista Install locks up on me.

Ive tried to install in Safe Mode, Diagnostic Mode (used run-MSconfig) as well as just system files, and almost every configuration possible with MSConfig, even trying to start up with the Vista disc in the tray. When I try to install using Safe Mode or run right from startup, it says I need to be in the current windows in order to upgrade.

It will lock up randomly, but always after I click next to proceed. Anywhere from where I enter the product key, to choosing which hard drive to install it on, choosing whether or not to do an upgrade, or clean install, but most often it happens when I click next at the Microsoft Software License Terms. Ive only gotten to choose a hard drive to install on once.

Is there anything I can do to install it? I know its a legitimate copy, I ordered it through HP, and it came completely sealed, so theres no doubt in my mind its legitimate. I dont care if I lose all the data I have on the PC, its all backed up, and I have all the programs on disc still, so reinstalling everything, although a PITA, isnt an issue. I just want to have Vista installed on my desktop. I installed it on my Laptop (Vista Ready) without a single problem.

If anyone has any suggestions on what I can try to get the install to work, Id greatly appreciate it! Any info you may need, Ill post up as soon as I can.

And BTW, the XP Media Center I have is legitimate as well, it came preinstalled on the PC when I purchased it from Best Buy, and have never had any problems with Microsoft to determine if its legitimate software.