I have a problem with Sony Vegas and how it handles .avi files

[On A Mission]

Hello. I had Sony Vegas and it handled .avi files perfectly, I had made a video which was mostly made up of .avi files.

Then I had problem with codecs and now Vegas doesn't handle them and it says 'media offline'. It plays the sound but not video. But .avi files work fine when I play them! I tried uninstalling and reinstalling vegas, plus the codecs but to no avail.

Only difference I have seen outside of Vegas is that the thumbnails in folders do not show up for .avi files anymore. I think whatever would make them show up would possibly make them work in Vegas too.

Does anyone have any help?