Why this emerged now after all the time I've been using Win 8 I don't know, but here's how I resolved it. The Security tab of my Documents folder showed an "account unknown" with a redundant GUID that wouldn't resolve. The same was true of the
other system folders I had moved over to my D drive, namely Documents, Music, Pictures and Videos. Obviously a consequence of reformatting my C drive. So firstly from one of the folder properties I deleted the account. Then on the Location tab I moved each
of the folders to temporary locations, and in the process the original folders were deleted. After that I moved them all back to folders with those original names. Since then WLW has been 100% stable.
Now to fix another computer with the identical problem...
Cheers,
Earl