MKVs to preview in windows explorer. I'm assuming it's 'cause they're huge files (1280x720p and about 2.15 GB each)
Mine are between 2-5gb for 720, Can you check under haali, that explorer integration is set to yes and the offset is 12000.
The other thing tha may screw this up is Windows Media Players database, might be worthwhile deleting that as can affect windows thumbnails.
You cannot view, add, or delete items in the library in Windows Media Player 11
I just deleted the lot shown.