my xp keeps on re starting

Hard to be specific when (a) very little info is given about the symptoms and (b) this problem can have so many causes.

Does the laptop reboot at random, or is there a definite trigger, ie every time you run a specific program?

Has it always behaved like this, or did this start happening only recently? If yes, have you just installed new hardware, device drivers, done a firmware update, etc?

If you recently made a change to your setup on XP, and this is when the trouble started, use System Restore to undo those changes.

Anyway, impossible to give you a specific answer without knowing more about the problem.

If all else fails, pay a qualified IT person to fix it for you.

You need to supply more specific information.

However, if your hard drive was not detected using another system, then maybe you have already found the cause of any problems.

I think heat. I see many laptops reboot or shutdown or crash because they are getting too hot. This is why newer laptops have the vents on the sides and not the bottom. My laptop is constructed like this as well.
You may want to look at some laptop coolers. They are cheap. I recommended one particular one which helped 2 laptops since even I could not find anything else wrong.
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1047073&CatId=607
Look for something similar but I would say give one of these types of coolers a try.