That shouldn't be happening and UAC shouldn't be altered from its default settings except under exceptional circumstances. I can only wonder if McAfee was installed in the first place by a user with Adminstrative credentials?
Also are there other security programs installed?
That's assuming this is Windows 7 SP1 and fully updated including IE9 (McAfee uses IE regardless of your browser preferences).
Try the Virtual Technician to see if it will fix things: http://mvt.mcafee.com/
If not try the following using an Admin-level log in:
Uninstall everything via Control Panel > Programs > Uninstall a Program
Run the MCPR cleanup tool listed under Useful Links at the top of this page and reboot.
Reinstall from your online account.
If all that fails then Technical Support is free of charge and available by phone or online chat through Useful Links also.