once youva backed everything up its as simple as sticking the disk in and restarting. Then it will say "press any key to boot from cd" or similar, press any key, then follow the instructions. If you have made a backup partition in disk managment then make sure you dont format the backup parition, as you will lose everything. XP will then format, then automatically install. Also, do not quick format as quite often it doesnt format correctly and the install fails, meaning you have to start again