Good luck with that. Regardless of the problem or the solution, back up your data at your first possible opportunity.

Sure thing, thanks.

This is the most bizarre computer error I have ever seen. I hope that works.

Yeah, it's certainly a new one on me too. It really could be as simple as drive letters being reassigned. As to what the root cause is, I don't know. Partly because I don't know what it would take for the system reserved partition to be able to have a drive letter assigned to it.

I used a simplified procedure and it has booted!

Not everything seems to be working fully (some things did not start automatically, I may have to reconfigure them), but at least I can boot it now. C: and D: are back where they belong, and all their files are fine.

I used the bcdedit procedure found here:
http://www.techsupportforum.com/forums/f217/solved-system-reserved-drive-letter-problems-544864.html

Except I didn't have any errors after restarting, it booted just fine. Now, to find out if something may still be wrong.

Great!

Which is why I again stress that you should get your stuff backed up right away. Without knowing what caused it you can't rest easy just yet.

After a second reboot, now everything seems perfect. But you're right, backing everything up is top priority now. What a scare.

If nothing else it has served as a reminder of just how important it is to keep backups. I know we all let that slide from time to time. It's human nature. You get comfortable and think that because nothing has gone wrong for so long it's not going to go wrong now. But naturally that's when it's most likely to.