Well, I used to play the GOTY edition on my computer b4 it got a NASTY virus and then i had to get windows 7 afterwards and fallout 3 got erased, so i downloaded it again and i got the first disc playing decently, but i just tried downloading the second disc for the downloadable content and all i get is the pc trying to run it, and the thing where it said to start started, but then it froze and wont DO S___! Can someone plz help me on this?

Boot into safe mode and install. (Also when getting the content from the disk its called installing, not downloading. Downloading is when you get it from the internet)
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx?mfr=true Yes, it says XP, but it will work on Vista/7 too.

It's because the executable file is over 2GB and it takes a long time (10 min or more) to load. Known issue with the GOTY disc 2. Just wait, leave it. As long as you hear the drive loading, you're golden

Booting in safe mode is always an easier alternative. It loads almost immediately.