If you are running Windows Vista or a later version of Windows, LOOT (and other modding programs) may be prevented from working correctly by the UAC security feature. There are four common workarounds to this problem:

1. Install the game outside the Program Files (or Program Files (x86) if on 64-bit Windows). UAC prevents unauthorised edits to these folders, so by installing the game outside of them, you remove it from UAC's reach, allowing you to use mods more easily.
2. Deactivate UAC. This can be done from the Control Panel, and will turn UAC off across the whole of your computer. It is up to you whether you feel that you have adequate security measures in place to do this without risk.
3. Give yourself "Full Control" permissions over your game install folder. This will allow you to make any edits you desire while keeping UAC active and on guard for any edits made by programs you do not run, and also means that you do not need to reinstall your game to a new location.
4. Run LOOT as an Administrator. Right-click the LOOT executable, or the shortcut if launching from a shortcut, and select "Run as administrator".

For those that wish to take the third option and give themselves "Full Control" permissions, here is a guide:

1. Right-click the folder you wish to change the permissions for, and select "Properties".
2. In "Properties", select the "Security" tab, and click the "Edit..." button. A UAC prompt may appear, simply allow yourself to continue.
3. In the "Group or user names" box, select the "Users" option.
4. In the lower box, check the box opposite "Full Control" in the "Allow" column. Press OK to exit. If a UAC prompt appears, allow the change.
5. In the "Properties" window, select OK to exit. You should now have Full Control permissions over your chosen folder.