post up what your specs are to see if there is anything there that may be the cause. to get it to boot from the cd it needs the sequence to be cd rom, hdd, floppy. another point, is the disk a full copy of wins or a recovery disk and did you have xp running on the system before?
BTW, the disk is original full copy of XP. It was running XP previously. Theres a sad complicated tale of disaster which I didnt think was relevant if I changed the HDD but Im starting to think it may be. Shall I explain?