OP SYS Serial#

Greetings! First post! My friend and I both purchased PCs with WinXP already installed. It's been two years now..and I have updated my Media Player, etc. without any Problems. Usually, before installation of anything..
Microsoft checks the Serial# then installs. My friend, on the other hand, does not seem to have the proper documentation on his hard drive. Can anyone suggest a prog/method by which he can find the needed # on his hard drive? I should mention that he purchased his PC from a local shop.
Any suggestions/advice would be appreciated.