Microsoft help

Help needed guys! A customer has had his PC stolen. As he is strapped for cash I have put together a machine from spare parts for him but he needs an OS. He still has his XP Pro CD but the serial key is still stuck on the side of his PC – now residing in some thieving a******’s living room. My question is: do you think Microsoft would be able/willing to help? And of so, can anyone suggest any telephone numbers to try? Thanks guys.