Recently, I swapped out the last machine I had on XP, a 5 year old laptop, to Vista. I even got my wife a Vista box, and things went mostly smoothly. Until she came to me one day and said things weren't working.

I dug around for a while, and tried to get into the management console as admin, sure enough a UAC dialog beep sounded, but there was no dialog asking for elevation of privileges. That's when things went crazy -- there was no dialog, but the system was still waiting for input. The screen didn't dim with the protected desktop, and the app that was asking for permission was hung. Alt-Tab worked except for the hung tab -- but no windows/apps could be closed/exited.

I pulled out the standard tricks to combat the situation, all failures, and finally went to safe mode. Even that failed to pull up the dialog. It wasn't drivers, it wasn't registry. Then came the last straw. I restored a backup of the machine from before the problem -- it didn't work!

Gave up for a night, and the next day she mentioned: "Oh, and the date's wrong". So I figure, ok.. i'll just change the date. But you can't because it requires UAC! I was fed up and booted into BIOS to fix the date, laughing -- at least I can fix this, I thought.

The punchline?

Changing the date fixed UAC. The machine was set to a year in the future. The worst part of this is that the PC should not have known it was a year in the future... is Windows phoning home on UAC requests? What's really going on with the time/date settings?