The below is me mostly copy and pasting what I posted in another forum for reasons I explain in the last section. Fix that worked for me:
1. Install this thing: BabelSoft / ????
2. Once its installed, open it and go under file formats(it might go there for you), check all the video formats that you don't get thumbnails of in folders.
(For me I checked everything under flash video, MPEG-4 Part 14, and Windows Media)
Check if it worked
-Go to your video folder with the video files previously causing dllhost to explode, and the green bar thing at the top should finish, thumbnails should show up (if they don't go check off more things with the method explained above in 2.), and dllhost shouldn't explode.
I had a similar problem (same problem of dllhost taking up tons of memory), and found out that it was apparently caused by windows epic failing at making thumbnail previews. Thus, after hours of installing crap and trying other 'fixes' that didn't work at all, I tried downloading the extension linked above that previews thumbnails. I proceeded to do what I listed in the instructions in the top area of this post and dllhost didn't explode when I went into the folders that previously caused it to. Also, all of the thumbnails that previously would not load loaded, with the exception of one, but nonetheless dllhost didn't explode for that one, so I'm happy.
Posted here cuz this thread came up a lot when I searched for this problem, I've posted in the other thread that came up a lot for me when I searched for this problem too. If the fix works for you guys as well, please go ahead and post it in other threads that came up a lot for you when you searched. I wasted way too much time getting this fixed. >_>