If it was taking 30 minutes to do anything, including booting, I'd be concerned that the hard disk is failing.
Have a look in the event log, see if there are any entries for bad sectors in there.
If there's some corruption on the disk, that might be the cause of Windows is no longer thinking it's validated, providing nothing else has changed.
I'd give the disk a surface scan to detect bad clusters and see what's going on.
Go to Computer, Right click on the C: Drive, and choose Properties.
Click the Tools tab and click Check now under Error-checking and tick the scan for and ettempt recovery of bad sectors
It'll tell you it can't do it while the disk is in use, click to confirm you want to schedule a disk check, then reboot the computer.
It may take a while, especially if the disk does have some bad sectors on it.
When it's booted, you will be able to see the result in the event log.
Good luck!
Ian
