Empty PhotoStream-folder on Window
Basically, you need to reset the iCloud settings. There are different ways to do so. 


Try to close the iCloud control panel and then quit all Apple and iCloud processes by using the task manager. If you reopen the iCloud control panel, chances are good your photos will start syncing.
Otherwise try to uncheck en re-check the PhotoStream option in your control panel (don't forget to hit the 'Apply' button).
A last solutions is to uninstall and re-install the iCloud control panel, which will reset the iCloud settings as well.