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.