I think you need a copy of xp home to rectify this, shouldn't have to be original as you are only using it for a repair and even if it comes to re-installation your licence sticker will register it as original. I think that the recovery disks will only work with the spec of the original pc as i have had this problem before. I don't think that the disk can be altered but i may be wrong. If you still have no joy i can get hold of a copy of xp pro with codes which i can post to you if you pm me your address. But doing it this way you will lose all data you currently have unless you can get another HDD boot it with WIN 98 or ME floppy, with your original HDD as a slave and copy off any relevant files. Then switch them back, install XP on the original HDD and all your files will be on the slave then simply copy them back across.
Unless anyone else knows another way, not sure if my way is the best, just the way i would do it.