Insert your XP CD
Reboot the PC and enter the BIOS {generally by the DEL key at the POST Screen}.
Look for a section that says "Boot" or similar. Set the CD as the 1st boot device, then the HDD.
Save & exit.
Now it ought to start Setup from the CD. After a few minutes, you'll get a few prompts, they are pretty easy to follow. It will then find the old copy of XP, and ask if you want to repair or replace it - you want to replace it. After confirming you want to do this, it will come up & ask you if you want to Format.
It's always a good idea to do a full Format {not the Quick one} and NTFS is the preferred file system. The Format will take a while, depending on Hardware & Disk size, then Windows wil begin installing.
It will have a few more Prompts for you - Timezone, Language etc.
It's pretty straightforward if you follow the prompts!
Good Luck.:wave: