how do i install xp on to a computer that already has 98 installed.

is it a xp upgrade cd or just "windows xp"?
also, for xp, you need to check that you have atleast 256mb of RAM (this is the min for xp) im not sure about processor compatibilities.
If it is just a win xp disk (not an upgrade to xp disk) the only wat i can think of, is enter the BIOS menu, navigate to "boot sequence" and click enter, move your CD drive to the top press enter and then ESC and click save changes and exit, reeboot with xp cd in the drive and follow the instructions for what you want to do (i.e upgrade to xp) you may need to format your HD first, although xp should do it automaticaly and remember to BACK UP ANY IMPORTANT DATA FIRST!!!

Do you want to replace 98 with XP or use both?

If the former just boot the XP CD & follow the prompts.

http://michaelstevenstech.com/cleanxpinstall.html

If the later read the following on setting up a dual boot.

http://www.microsoft.com/windowsxp/u...multiboot.mspx

As mentioned above make sure that your system is compatible:

http://www.winsupersite.com/showcase...p_sg_clean.asp