It has gotten tougher in recent times. However it is worth a try since you have the installation key. First you need a copy of XP on disk. It should include SP2. Nest make sure the computer setup is set to boot from CD. Start the computer. The installation process should begin.

Sometimes you just can't get the installation to start. I have always suspected that it is due to anti-piracy crap that M$ has included in updates. When this happens the only thing that I have found to work around it is to download the diag utility for your HD and write 0's to the disk. This filles every space on the disk with a zero. Try the installation process again.

Once the installation is completed, you will have to validate the installation. It may go through without a problem. If not you will receive a message that your copy of XP appears to be counterfeit and it will not be activated. At this point you can try to talk to M$ but I don't think it will do any good. They will tell you to go to wherever you purchased the computer for support.

You have a chance. All it will take is time and effort to find out. If it fails, you won't be any worse of than you are now. Good luck.