If you have a working windows xp cd, then it will have boot support on it.
The problem you're having is that your BIOS boot is probably set to 3.5" drive or Hard drive before the cd rom.
What you're going to have to do is to go into your motherboard BIOS (not physically) and change the boot startup settings. Depending on the type of motherboard you have (see your manual or try F2 / DEL / F10 / F12) when the computer is starting up, it will tell you to press a key for setup. Once you get into that, look for Device startup order. All you have to do is move the cd/dvd rom to the top (1st) and you're good to go (make sure you save the settings before you exit.
Put in your windows xp disk and reboot the pc and you should get teh "PRESS ANY KEY TO BOOT FROM CD ROM...." or some crap like that.
I hope this is helpful :)