1. Back-up all your files that you wish to keep.
2. Make sure you have all driver CD's
3. Make sure you have all the CD keys you need
4. Insert the Windows XP CD into the CD drive
5. Restart the System, it should then boot from the CD.
6. Follow the instructions on screen.
7. When it gets to the format options list, select NTFS (not quick)
8. Then XP should install, then it will restart, go through a quick setup.
9. Then you should be back into windows and be able to loads drivers, games ect. back on to the computer. then you should be done.
I think i have covered the main parts there that you need to know.