Eu tive um problema dias atrás com um arquivo mp4 que não consegui excluir da minha área de trabalho, mas não sabia que era o problema.
Encontrei esta resposta de answers.microsoft.com :
The problem appears when dllhost.exe attempts to create a thumbnail of an mp4 file, but for some reason cannot complete the task, and starts reading the hard drive non stop and filling all phisical Ram, and starts using the swap file (virtual memory file wich uses hard drive space as ram memory), slowing down or freezing the computer. The cause is probably an error in the integrated mp4 windows codecs.
Anyway, the solution is to download and install a freeware program wich generates all video files thumbnails instead of windows. It even creates thumbails of flash (flv) files!
You can download it from this link:
http://www.free-codecs.com/Media_Preview_download.htm
Or googleing Media Preview 1.3.1.343