Have you installed any software recently or downloaded any files off of P2P FastTrack or Warez websites? (Kazaa, limewire, etc.). Sometimes this is caused by virus's, spyware or corrupt system files.
This could also be caused by if you're caching thumbnails or the view you have it in and it's a slow computer. Typically this is severe, often requiring reformats and reinstalltions of the Operating System, if it can't be fixed by other simpler means.