So i used to have fo3 on my computer until i cleard out my hard drive. i reinstalled the game but i need the serial code and cant find it. i lost the game case and manual a while back. Anyone know of a way to find my serial?

There is no need for a serial number to play the game. The code is only needed for Games for Windows Live. (Which you really dont need). Other then the GFWL code, there is no other serial number, only a simple disk check is DRM.

I had all the dlc's before and i need to register again with gfwl to use them.

I can only suggest you to contact Microsoft Games for Windows support (Finally they have a support line) and tell them your situation, but you will have to verify that you own game. they may be able to find your Live key or issue a new one.

I don't remember the number, but they do have one.

Or you can move your DLCs so you can play then without GFWL. Should be in one of the stickies.

If you ever entered a GFWL key into the system it will be on file with them. Other than achievements having games for windows live is a complete waste of time. If you have to format your computer and reinstall everything there will be no trace of your saved games even after reinstalling GFWL. I am still trying to figure out why the steam version of fallout needs it since you can download the game from their site and don't need the disks.