ok when you start your computer it should say press del to enter your BIOS or something. Press del or whatever key it says and a new screen will appear.
It might be a little complicated but look through some of the settings and find something that says boot order or something like:
floppy drive
hard drive
cd rom drive
etc
you want to move the cd rom drive to the top so click on floppy change it to cd rom. then put your windows xp cd in. Exit the bios make sure to save changes.
When you start up your pc now it should say press any key to load cd... do it. Then the xp cd will start up.