Well you have to check if your vista drivers are xp compatible.

Drivers in general are not always useable in both OS versions.

Software is not a problem mostly.

But drivers need to be Vista or Xp certified or at least that the company says it should be compatible.

I have never downgraded so i have no personal experience, only upgraded.

If you cant find it out i`d suggest contact your computer brand company for support.

Btw: I have an HP and i had vista 32bit installed, but maya crashed with large files so i upgraded to 64bit. HP is not great in support as there pretty stupid. They claimed that they did not support drivers for it, while they have drivers online. So there a bit outdated on support, Anyways why i`m saying this even thou a company dose not support an OS version there are always ways. As HP at the time did not have all drivers so sometimes you need to go to the manufacturer itself like Mainboard, cpu, sound, etc etc...