Nige when your trying to boot from the xp disc you have make sure the cd drive is disconnected on both power and ide leads. Some bios will only let you boot from the drive that is on the top end of the cable. If this drive is duff it could be locking the system up.

The last thing I would try is swapping the hard drive onto the dvd drives ide channel, this would determine if the board is duff as the hard drive wouldnt boot now.