"File could not be opened"

(In Sony Vegas Pro 8.0)
When I try to import a video into Vegas, it says "File could not be opened. Make sure the file exists and that you have acces to the file/folder." Well, I think I do have access, if I play these videos, they work without problems. The format is .mp4 . Point is, I have 2 videos of the same format that do work, and I don't see what could cause the difference between these videos, having access and not having access. They're even in the same folder!

Next, the .avi-files that I try to put in, just give me audio and no video. I checked the forum for this problem, and found it could have something to do with "Mute all video", but that one is turned off. The "no video" already starts in the Project Media part, it just shows a .avi-image instead of the first frame of the video.

Which formats do work anyway in Sony Vegas 8.0?

Help me please, before my AMV-inspiration is gone again =D