The problem is that sometimes when you connect a camera to a Windows 7 computer, it won't download all the pictures because it *thinks* it's already downloaded some of them. And that may be true but maybe you just want to redownload them anyway. Well here's how. Simply rename the file, "C:\Users\%username%\AppData\Local\Microsoft\Photo Acquisition\PreviouslyAcquired.db" to "..\PreviouslyAcquired.db.old". And that's it!
I found this information from here: http://www.ardamis.com/2009/12/09/fixed-no-new-pictures-or-videos-were-found-on-this-device/
