Lester, this is how I go about it.
After I take my avies I create folders for each avi with the avies name on it. I then put each avie in its appropriate folder. I then open Virtual Dub and Windows Explorer so I can access both for drag and drop. I open the first folder and then drag the AVI onto VD. I then go File > Save image sequence. A dialogue box opens. Ignore the top 3 lines of options. On "Directory to Hold images" navigate through the browse button at the end to the directory that the avi came from. (BIG HINT: When naming your directories, keep all their names the same except for the end. Use a consecutive numbering system like "jupiter 19-25". ie Jupiter Avi shot at 19:25. If the next avi is 5 min later then "Jupiter 19-30".
Next make sure the "window bmp" bullseye button is checked and the "Don't run this job now" box is checked, if you have a few to do.
Select OK.
Go back to window explorer and select you next avi from your next folder (hopefully Jupiter 19-30" Do the same as before, File > Save Image Sequence. When the dialogue box opens up, rather than navigate to the folder you need simply remove the 25 from the path and file name and substitue 30. If all your folders are in the same sub-directory and have the same names except for the ends then it saves a lot of time by simply editting the end of the file name rather than re-navigate to the required folder. Keep the bmp checked and the "Don't run this job now" checked.
Click OK and repeat until you have batched all your avies. Now go to File>Job Control>Start and go to bed for the night. When you get up in the morning all your Avies will be converted to bmps and stored in their right folder