Ulead Video Studio 10 - only audio after converting?

Hi! I'm trying to use .AVI files in an AMV I'm making using Ulead Video Studio 10. When I originally imported the videos, they appear in the preview box, but whenever I tried to put them in the timeline, I got the message "The Content of these Files is Unreadable:." I know these are not bad files, because I've used them in WMM and a few other video editors before. Besides, it showed up in the preview box. Somebody told me to try using the converter built into the program - I did, and converted the file to both .WMV and .AVI again. Both times, these files will go into the timeline, but only the audio will play - the video doesn't show in the preview box or on the timeline. Can anybody help me with this? Thanks!