Just bought Fallout 3: Game of the Year Edition on Steam.

When I run the game, it asks me for my GfWL product key.

Where can I find this?

Did you email Steam support?

It shouldn't ask for the key anymore, only install the client. Are you sure you can't close that window or something? There's a http://www.nexusmods.com/fallout3/mods/1086/? if that's the only option.

I should try.

I can press Home to close it but if I find a product key, I'll be able to earn Games for Windows - Live achievements, won't I?

GFWL is no longer active as far as I know. The game only needs the client installed.

After rumors of Microsoft shutting down the service last summer, most developers removed GFWL from their games. In the end GFWL stayed active, only the markeplace was closed, but Valve stopped giving out new keys for Fallout 3 and removed them from copies bought prior to the change. You might be able to retrieve the key if it had been issued before July 2014, but I doubt Valve support can give you one for a new game. If you really want to activate it, using a product key from any other legacy GFWL game should work.

Don’t worry, you don't need one! Fallout 3 and Fallout 3 GOTY Edition purchased on Steam do not require a CD Key.

If you are prompted to enter a CD Key when you start your game simply log out of Games for Windows Live and decline the Register CD Key prompt to proceed into the game to play.