Boot up the system from CD directly, NOT YOUR HDD. This can be done via your
BIOS configuration. Typically , soon after you turn your system on, just
press F2 (could change based on your computer/motherboard manufacturer).
When you boot from the windows XP cd/dvd, you will be allowed to install the
fresh copy.
I never tried this, but one other option is to punch in the serial-key
directly and see if it works.
Else, you always have MS support .
Thanks,
Sashidhar