I have a decent xp tower and It has a tiny hard drive witch I thought would be easy enough to replace with a 500 gig or higher. I do have my xp op disc but how do I know what the correct hard drive is to purchase ? I actually have two 500gig ones but when I connect one and tried to boot with xp disc in the system, it didn't find a drive ? can anyone advize please. Point of interest "the 500 gig drive has vista op on it at the mo" Actually the two 500 gigs were out of a tower of mine witch were both installed making that one 1 tb 1000gig surley I can use one of them ?