I recently bought an old ex goverment pc with no operating system on it, they provided a copy of windows XP, on boot up it requires a 25 digit code, which was not supplyed. The disc is new, and states on the packaging only to be sold with new P.C's and to find the 25 digit code on the side of the computer, as it is an old P.C it's not there. Is there another way around this?

No.

That OEM disk was meant for one computer and apparently not yours.

Unless the government can offer you a Product Key to go along with the CD that CD cannot install XP.

Sorry.