Had this problem when I was using win2k.
When I drag avi files to the timeline I only get the audio. I'm pretty sure that last time I installed ffdshow codecs and all was cured. But this time it's not working.

Most likely a different codec then! Seek, download and install a program called GSpot (free). Run that on your avi and it will tell you everything you need to know about the codec used (plus plenty you don't need to know) - or rather, it will at least identify the codec.

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Tim