Reminder: Windows Vista SP1 RC expires on June 30
Starting June 30th, any machines running a release candidate of Windows Vista SP1 will only be able to run for about an hour at a time. Basically, you boot your system, it runs normally for an hour, and then the kernel shuts down and you get a message that says END_OF_NT_EVALUATION_PERIOD. At least it's fairly easy to understand.
But wait! Didn't Microsoft push Windows Vista SP1 final out through automatic updates? Well, yes, it did. But you can only install Windows SP1 if you've first rolled back any release candidates of the service pack. So if you haven't done that yet, there's no time like the present.