The only thing I can think of then in WinXP that lets you run stuff under a different account without having to enter the password every time are services - if you can somehow make a service to run that game under desired credentials, and give him a shortcut that only starts that service (dunno if starting would require admin though, so defeating the purpose), then that might be a messy workaround for you Otherwise not having to enter the admin password is a bit of a security issue

Otherwise, image the machine then give him admin, so you can restore quickly and easily when he screws it up