My son brought home a used 03 Hp desktop that wouldn't even run my XP to do an install. I had to order a replacement CD from HP but I ended up doing a complete rebuild after all with new MB and CPU so then I could only use my person XP CD to install .
I think many of the name brands use proprietary software.
Something in the MB or bios that is specific to the brand- doesn't let you run your personal Windows CDs to install or re install.

You might be able to copy/install the XP as a secondary boot system and use it that way{ not sure though}then leave Vista on it but don't use it??

I did set my aunts new laptop {Toshiba/ Vista}up for her and changed all settings to Classic look but she isn't a heavy pc user - but she hasn't called me with any problems.