A local computer shop installed Windows XP PRO on a friends PC but did not give him a disk or license (they did not charge for it either).

When I try to run updates it starts to install the Service Pack but says the product code is invalid.

I have contacted Microsoft etc but need to get this working.

If he buys XP PRO can the software be re-installed with the valid key that comes with it WITHOUT having to format the system and start again?

If so how?

Thanks