use one IDE cable, plug it into to channel 0 on the mobo, (or which ever one you want doesn't really matter) i presume you know which end goes into the motherboard (the one with the most distance AWAY from the centre slave IDE socket).
then set both HDD's to cable select via the jumpers on the back of the HDD, and then the HDD with the OS (Xp) on, put that on the ending IDE connecter on the opposite end of the IDE ribon cable plugged into the motherboard, then plug the secodary HDD onto the left over IDE connector in the middle of the IDE cable (the slave connector).
Then boot into BIOS have a look around (i dont know what your BIOS is), and then make sure that both slave and primary are enabled on which ever channel you have plugged the IDE cable into (either 0 or 1), infact just enable all of them shouldn't make a difference.. then you set to go, oh and also make sure that you set the HDD with the OS installed on to 1st in the boot priorty, if you have the option to in the BIOS that is ..
hope this helps