further guess.

I have laptop (natebook) with Win XP with sp2.I have formated the HD to 3 regions, C, D,E. It works well. problem appears after I installed Vista Ultimate with sp1 to the D region. The XP os no longer worked stably nor correctly(no sound when play video) and it was totally down finally .
The problem appears: The presence of Vista prevent me from freshly installing Win XP with sp2.
I toke out the Hard diver and put it in an enclosure (USB),back up the Data files; format to C, D, E to totally erase the OS.

1. If the Vista OS comes with your laptop or PC, you may lose the chance to back to Vista if you erase the Vista from the hard diver. In this case, use other hard driver for the fresh XP installation.
2. format your Hard driver into 2 or more regions, one for the OS, other for data and files. you recover your data via USB enclosure (now it very afordable) when the system is down and reformat the OS region, reinstall the XP OS.