I assume that you got a Win XP Pro CD in the CDROM.

Boot from CD:
Go to the bios settings. Go to "Advanced Bios Features". On the menu calld 1st Boot Device change it to CDROM. Quit BIOS by pressing ESC and ESC again. Press Y as Yes when asked to save or not. Wait for comp to reboot. Follow the instructions on screen.

Start the system through a boot disk. ( This require a boot disk that installs drivers to the cd roms.
When at the dos-promt, write this:

X:\ [Change X to your letter/drive]
cd i386 [Enter]
winnt [Enter]

Follow the instructions on screen.

The 1st method is the best one.