server 2003 licensing issues

Hi All,
I have a client who, after a bit of a mix up, didn’t recieve server 2003 media although he bought all the relevant licenses.
He ended up using a server 2003 cd that “magically” appeared. However he is unable to install any patches or service packs and recieves the error message “unable to continue because this is an invalid product.
The proper media has arrived but is it necessary to format the drives and reinstall the software. Or can I aviod rebuilding the box by some registry tweaks ?