Use the search bar to find where you installed Fallout 3.
Warning!!! The Search feature is moronic at best in Win 7. Way to go Microsoft advance the OS a little bit overall, but take 3 steps back in some basic features....
Type Fallout3.exe in the Search bar. Click the Windows logo in the lower left corner. Win 7 will give you "No items match your search". Click "See more results".
A window will open up to allow you to "search again in....", click Computer. Fallout3.exe should now show up and it will also tell you the path to where the file is located. Go to the game directory, then drop in the D3D9.dll file in there. Be warned though, people have complained about performance with the GMA 4500.
Microsoft... yer a dumbass...