Warning: An error occurred while opening one or more files / codec (Sony vegas)

[Taint]

The title says it all, When I import a video to sony vegas I just get that message:
Warning: An error occurred while opening one or more files / codec... !

http://i58.tinypic.com/jj4u2r.jpg

Help ? (Editors)

when did it start happening
what did you download in regards of plug-ins/cracks

was that video downloaded off youtube? was the video clip your own?
what format is that video clip you're trying to input