I told Process Monitor to monitor the system boot, and then rebooted.
The system started, but some some programs started freezing.
From exploring the symptoms a bit I suspected it was the antivirus, so I turned off Microsoft Security Essentials and told ProcMon to monitor the boot again, and then rebooted.
This time it booted fine, but then UAC suddenly got enabled!
Why would this happen?
It seems like a bunch of the Group Policies are also reset, not just UAC. :\