DivXRepair is a compact portable tool which will try to repair AVI files with bad or missing frames. (A sign of this might be a video which freezes when you try to play it back, or briefly breaks up into coloured blocks of pixels.)
The program is fairly straightforward to use. Launch it; click "Add file" to add your troublesome AVI video (you can't drag and drop this onto the program, unfortunately); click "Repair files" to launch the repair, then wait to see what happens.
If all goes well then a text report will appear explaining what (if anything) the program has done. And if this involves a repair then there should be a new "fixed" version in the same folder as the source (if this was MyVideo.avi, that file would remain untouched, while the program creates a new version called MyVideo.REPAIR_OK.avi.)
In our tests the program's results are variable, though. Sometimes it did fix video corruption problems, and very well. But on other occasions the program would do nothing at all, and a couple of times it started to write a new version of the video but then hung up itself.
This isn't some magic solution, then, and you may well be better off using VirtualDub (on which the program is based) or some other tool to fix the video manually. But the program is at least easy to use, and does work sometimes, so there's no harm in having a copy on hand, just in case.