if its win xp thats ez
put your xp cd in the drive and boot to it
just to make sure if you dont know how to boot to it reboot then start pressing f8 till you see a screen that says somthing like cd/dvd hdd network
select cd/dvd then when it says press any key to boot to cd, press any key
then you will see a blue screen (not the dreaded bsod) that will load some drivers
follow the instructions and your on your way.
hope this helped