I upgraded my motherboard/processor/memory and video card and I asked on here if I could get away without reinstalling XP Pro.
As I suspected, XP refused to boot, blue stop screen.
Booted from XP cd and on the first prompt, selected to install fresh copy. On second screen said to repair. Took ages but it went through and repaired XP and let me boot into it. Then no internet and few other problems. I installed all the new drivers first, reinstalled SP1 and security patches and hey voila, brilliant, faster etc and all my programs still there.