If the pc won't boot from your Windows XP cd (ie, put Widows disk in the cd drive and then reboot computer), try to get into the BIOS. (There should be a line or two as the computer initially starts telling you which key/s to press to enter the BIOS.)
Find something called "First Boot Device" or similar and make it CD/DVD drive then Hard drive or HDD second.
Save those settings and, with the Windows CD in the drive, reboot again.
If that doesn't work, big hammer.