Black screen on preview

Hi peeps, i have looked through some of the other posts on black screens but they are a little different to my problem. I use sony vegas for editing and just came across this problem all of a sudden. i can view the timeline video and sound no problem but when i try to preview the clips in the top left hand box i get sound but no video. i downloaded klite and the same happens with that (sound no video) i have also updated my video drivers but to no avail can anyone please help me?
running nvidia 8800gtx 764mb 4 gig ram 2tb hdisk quad core dell xps720. many thanks

I have the same problem myself. I am currently trying to find out how to fix it.