1) Back up all user Data
2) Verify you can get all the drivers for XP. Normally from MB Support website, or Computer Manuf support website.
3) Have ALL programs that you need to re-install handy, Yes All programs must be re-installed.
4) Download Ultimate Boot CD (UBCD), boot to CD and delete all partitions. http://www.ultimatebootcd.com/
5) Boot to XP Installation CD and Install Windows XP. If a New computer XP install Disk MUST have at least Service pack 1, perferably 2 or 3
NOW, for the question - WHY. Do you have a Program compatability problem or a Hardware problem.
Question 2. Are you changing 32 Bit -> 64 bit or vsa versa.
For Programs, the ONLY programs that have a compatabilty problem are 32 Bit programs that have 16 bit code can not be run in 64 bit Win 7. For the 32 Bit OS, I've run programs that were designed for win 98.