It's not UAC, as far as I know. Switching it off won't make any difference. Vista and 7 just have a really anal thing about editing files in the program files directory. This is why I almost never install anything there.
There is, however an easy way to do it. You can't edit the file and save it in there, but you can overwrite the file by copyin over it. So here's what you do.
1. Copy the config file somewhere else (your desktop, for example).
2. Open the file in the new location, edit it, save it.
3. Copy the file back into program files, choosing to overwrite the old one.
4. ...
5. PROFIT!
That should do the trick. It's what I've always done (when for some reason I had installed to Program Files rather than somewhere else) and it's always worked.