I use this program called movie thumbs, but you have to do each movie one by one. It makes the best thumbs though.
I don't know of any batch programs off hand for this. I like to do it by hand 'cause random captures can come out looking crappy. I know it's a pain, but it's def worth it.
If you're willing to pay for manual thumbnailing, try ->
http://webmasterlabor.com/
They've done work for me and although I wasn't too happy with some of it, they did a good job on capping and movie thumbnailing, so if that's all you need, you might wanna check 'em out. btw, they are dirt cheap.
Hope this helps,
Brian