Is there any way to re-download Photo Stream photos on a PC/Mac?
For the PC the way I got it to redownload the photo stream is first go to Control Panel then open up iCloud control panel.

Turn Photo Stream to off and close the iCloud control panel.

Just as a precaution, rename the old Photo Stream folder just to have a backup of what was in there before.  This is up to you.

Then go to C:\Users\yourusername\AppData\Roaming\Apple Computer and rename the MediaStream folder to something like MediaStream-old.

Then go back to Control Panel, open up iCloud control panel and renable the Photo Stream.

If you chose a custom download location for Photo Stream then you have to choose it again.  The photos should start coming in, but just remember it will only grab the last 1,000 photos you have taken.

Thanks.