I have upgraded my laptop to Windows 8 and all was fine for about three hours and then I started to get an error when trying to run any application that required Administrator privileges. Half an hour of Googling led me to this blog post Case of the broken uac prompt This problem was caused by downloading themes. The final theme I had decided on included a new sound theme. Clearly at least one of the sounds did not play (the one for the UAC prompt!). Changing the sound theme (right click on desktop, choose "personalisation") to Windows Default sorted out the problem. Doing a clean install would also have fixed it but that would have been a drastic solution and as soon as I installed a new theme it may have happened again.
Random thoughts and idle musings