It is definately not your CPU.
What is your full system config? what memory do you have? what memory does it say you have in BIOS?
Also are you installing both XP and Vista from the sames CDs each time?
Intel Core 2 Duo E8400 3.0GHz 6MB L2 Cache with 4 GB RAM Crucial 800 MHz
Tried to install XP from a separate CD the installed Vista different CD.