The Steam version of Fallout 3 is not optimized for Windows 7, like it would have probably made clear when you purchased the game on Steam. However, you are in luck. There is a fix to allow the Steam version of Fallout 3 to run on Windows 7:
Go to My Documents>My Games>Fallout 3, or for your Steam version it is probably located in Computer>C:>Program Files(86x)>Steam>steamapps>common>Fallou... 3. Open the Fallout_default.ini file with notepad. Look for a line that says bUseThreadedAI=0. Change that to say bUseThreadedAI=1. Then hit SPACE key and type in iNumHWThreads=2 right below the other line. If you still experience crashing change the 2 to a 1.
I had your exact problem when I bought the Steam version of Fallout 3, and I used this fix. It has run beautifully ever since, and I even run 10 or so mods on it! If you need to read a more detailed description of the fix, a quick search for "How to run Steam Fallout 3 on Windows 7" should gather some decent results.
A forum with the fix and more answers: