If you are wanting to start from scratch:
Insert xp cd into cd drive.
follow instructions to prepare the drive - format etc.
If, when you start the computer, it doesn't start running from the cd, then,
restart your computer,
press 'delete' key [depending on what computer, it will tell you to hit the xxxxxx key to enter setup]
This is the BIOS, be careful in here. You can't hurt it just by looking, and if you do change something you shouldnt have, just press EXIT WITHOUT SAVING CHANGES
Look for standard setup,
Change boot order - 1st device should be CDROM
2nd device should be HDD or hdd
SAVE and EXIT [might be F10, it will tell you at the bottom of the screen]
computer will restart, and the xp cd will start automatically.
Let us know how it goes. Good luck. Well done for doing this on your own - and also for managing to find the right place to learn more about your computer
