I’ve never tried to do this before, but if I were going to I’d search for info on Twain. You might check http://www.codeguru.com. They have a decent store of Win32 knowledge. I just did a quick search on MSDN and didn’t come up with anything. (The CD version, not the website.)
I did some searching because this is something I’d be interested in learning, as well. (I’m also intereted in getting data from web-cams and I noticed once that my Creative WebCam has a TWAIN source.) Here’s some links that might be helpful…