You're off topic for this forum, which is dedicated to Windows Live OneCare.
However, insert the XP CD into the CD drive and reboot the PC, assuming that it will boot from a CD. If not, you will need to determine how to access the machine's BIOS or CMOS where you would change the device boot order so that the PC will look for an operating system on the CD first, before loading the OS that exists on C:\
If the PC has less than 256 megs of RAM and a small hard drive, your experience with XP will be pretty miserable on that PC. Typically, a PC of that vintage will not run XP very comfortably.
-steve