Unfortunately, I have no restore points as this problem started as soon as I'd reinstalled Win 7. Strangely enough, when I go back to my original Win 7 installation, I have the problem there, but I didn't before reinstalling to a new partition. (My reason for reinstalling was unrelated to this.) The WP I'm using is under the control of TSF Admin so I can't touch it.
You can get the program for free from here: http://www.nirsoft.net/utils/shexview.html
Disable the 3rd party (non-Microsoft) shell extensions, reboot, and see if the problem has been fixed.
If it has, then go through the disabled items in groups of 5 until you find an offending group.
Then go through the offending group until you find the troublesome bugger that's causing this.