The single most asked question:
Q. I get the "Unable to get write permissions for fallout's installation directory" error. How do I fix it?
A.  This happens when you are running Vista or Windows 7 and have installed  Fallout 3 in the Program Files folder. You need to do one of the  following:
- Disable UAC (not recommended).
- Move  Fallout 3 outside of the Program Files folder (e.g.,  C:\Games\Bethesda\Fallout 3). This may require a reinstall. I know with  Oblivion you could just copy the game folder to a new location, run the  game, and all would be well. I'm not sure if that works with Fallout 3.
- In  theory you should also be able to run FOMM as administrator. You can  try this by right-clicking on the FOMM shortcut and selecting "Run as  administrator." Alternatively, right-click on the shortcut, select  Properties->Compatibility and check "Run this program as an  administrator."
The best thing to do in order to avoid  other problems, and the generally recommended solution, is to install  Fallout outside of the Program Files folder.