Vista Tutorial - Movie prewies (thunbnails)

Hi!

In the beginning of my vista use (2 weeks ago) - Vista gave me a thumbnail
on almost every movie i put in my video folder (and it is alot of files, i
use i digital videocamera in my work) anyhow, this has suddenly stopped, i
just get an "mpeg" picture and no thumdnail prewies anymore, they helped me
alot when organizing my videos. Is this a function i have by misstake turned
off, and if so - where do i put in on?
If i remeber it right it was a green progress bar in the adressfield earlier
when the folder was "analyzed"
Thankful for any answers....
Niklas