It started yesterday, and I shut it down before today, does that count as restarting?
It does.
Also, I don't really know how to fix this, but I can tell you a few things to try:
In Folder Options, try hitting "Restore Defaults" in General. (might be different in 7, but it's at the bottom of the window in Vista).
Try selecting "Open each folder in it's own window", hit apply, then select "Open each folder in the same window" and hit apply.
I'll see if anybody else on the Internet's had this issue, and see if I can find some solutions.