Have you contacted the reseller whom you got the system from to see if they
offer the downgrade option?  They may help you.

Possibilities:
- Scratched/defective CD (Windows XP Installation CD.)  Try cleaning it with
a soft/non-abrasive cloth.
- Something is 'off-kilter' with the CD/DVD drive you are using (laser is
off, it's a poor performing drive, etc.)  Try cleaning it with a CD cleaning
kit and/or replacing it with another drive to install with.
- Bad cable from CD/DVD to controller - try replacing it.
- Defective hard disk drive - try running the hard disk drive manufacturer's
diagnostics (from their support web page downloads) on it - and since you
are installing XP fresh - a full zero-write on it is not going to hurt
(might help.)
- Defective memory/memory slots.  Find, download and utilize a memory
diagnostic utility to test the memory for a few hours or more.  If you have
more than one stick of memory or you can change what slot the memory is
plugged into and still boot - do so and/or swap the memory around (so it is
not in the same slots.)
- Bad (inconsistent) power.  Make sure you are plugged into a clean power
source.  Something like a UPS with a Voltage Regulator would be ideal.
After that - it could be that your power supply is defective.

NOTE: usually it is more likely something from the top part of that list is
the culprit - not the latter part - but it all does happen in the situation
you describe.