Problem downloading photos from iPhone Photostream to Windows
It seems that Apple dropped the ball with the new iCloud services for Windows. I've had the exact same issue on 3 different machines and iPads/iPhones from different family members.

What worked for me:


Make a backup of your PhotoStream folder on your PC.
Open the Windows Task Manager (CTRL-SHIFT-ESC) and kill the following processes if they are listed:


iCloudService.exe
iCloud.exe
AplePhotoStreams.exe
ApplePhotoStreamDownloader.exe
APSDaemon.exe

Then go to the following locations and delete the whole folders (or move them somewhere else for backup):


Your photostream folder:
C:\Users\&lt;YOURNAME&gt;\Pictures\My Photo Stream
Mediastream Data folder:
C:\Users\&lt;YOURNAME&gt;\AppData\Roaming\Apple Computer\MediaStream

Then run iCloud directly:
C:\Program Files (x86)\Common Files\Apple\Internet Services\iCloud.exe


In theory, things should sync again with the following caveat:


Only pictures taken in the last 30 days will be synced.
At most 1000 pictures will be synced (if you took that many in the last 30 days).
Try some of the other solutions from the Thread My Photo Stream is empty with iCloud Control Panel 3.0 from the Apple forums.