I'm on Windows 7 64-bit and every time I start TIM 3 I always get the security warning asking if I want to allow it to make changes to the computer. Normally you just have to say "Yes" once and that's it.

Right click the shortcut and choose Run as Admin' instead of double clicking it.

Or if you want a more permanent solution tick the 'Run as Admin' checkbox on the shortcut

(Right click the shortcut >Properties>Switch to the Compatibility tab)

Hmmm nope that didn't do it I'm afraid.

Well, short of disabling UAC entirely i'm not sure then. Assuming off course that the warning indeed comes from UAC.

Try reinstalling the game outside of the program files folder to something like C:\games.