My friend has an XP reinstall disc from gateway, and hes switchin to new mobo, proc, and usin the same hd from the gateway. he's going to need to format the drive, but can he reinstall xp from the gateway disc onto the new setup?
Well you will be able to install it, but you will not be able to activate the product since you have changed key components. You will either need to buy a new activation licence or contact microsoft and beg them to give you a new code, just explain that you have upgraded your mobo.
Depending on the model/manufacturer some restore disks won't even let you install, they look for a codekey image on the MB that the manufacturer puts there. All you can do is try and see...
If he's just getting a new MOBO/Proc, then he might want to make absolutely sure that the old PSU will work. I've heard several instances of Dell making products that only work with other Dell products (particularly PSU's).