Why does Colin McRae DiRT stop working?

My computer is an old Dell running XP SP3. Recently, while trying to play DiRT, the game hangs as it tries to load a race. That’s pretty mich it; I’ve tried all different kinds of cars, tracks, liveries, it’s all broken. Cleaning physically the CD didn’t do anything. I would reinstall it, but I can’t find that damn product key. This is maybe the 10th time I’ve lost one and I’m sick and tired of it What should I try now?