I have an bash/ffmpeg script I wrote to extract thumbnails every X seconds off of video. I never got it to work on wmv files but I'll bet with some trying you can make it work if you absolutely need it (right now it just extracts from MPEG-4 files). Other than that, frame-shots.com has a cool tool for grabbing screenshots from video files, that's what I use for my WMV files. Hit me up on ICQ if you want that bash script!
