MS already have UAC in place, ELUA clauses which state "you are prohibited from modifying the O/S to overcome technical limitations"
You think that's bad, here's one from iTunes: THE APPLE SOFTWARE IS NOT INTENDED FOR USE IN THE OPERATION OF NUCLEAR FACILITIES, AIRCRAFT NAVIGATION OR COMMUNICATION SYSTEMS, AIR TRAFFIC CONTROL SYSTEMS, LIFE SUPPORT MACHINES OR OTHER EQUIPMENT IN WHICH THE FAILURE OF THE APPLE SOFTWARE COULD LEAD TO DEATH, PERSONAL INJURY, OR SEVERE PHYSICAL OR ENVIRONMENTAL DAMAGE.
Hands up anyone who's using iTunes to run any nuclear facilities.