Currently Being ModeratedNov 20, 2011 12:45 PM (in response to HowardFromNorthVancouver)
After I posted this request for help I found the discussion "Photostream is not syncing on my Windows 7 PC" which gave me some ideas. The Photo Stream download to my 32-bit Windows 7 PC now seems to be working.
Yesterday I changed "C:\Program Files\Common Files\Apple\Internet Services\ApplePhotoStreamsDownloader.exe" to XP SP3 compatibility mode then killed this process, which was running at 100% of one of my CPU cores. I also configured iCloud to download to a new "download" sub-folder of the default Photo Stream folder.
It isn't clear to me whether all of these actions were necessary to solve the problem but when I took a new photo this morning and checked the download folder a few hours later, there it was! Also, Downloader.exe is not running continuously as before - it seems to be a client application called by ApplePhotoStreams.exe only when there is something new to download.