Due to SSD and recovery problems on my 8.1 Pro X64 system I was forced to go back to basics and reinstall Windows 8 and then perform the upgrade to 8.1, all of which went quite smoothly.
However I have now found a somewhat puzzling difference in how a search with "kind:=video" responds;
on my old system this search always returned all video types regardless of what was being searched, but now the results vary:-
if I search a folder or a drive then everything works as before, but if I search in a library the the search does not pick up MKV files.
It's a real head scratcher.