I have a friend who had been using a proprietary photo album product and reached a hard limit on the number of photos. He managed to export the data to spreadsheet format (including image filenames) plus he of course saved the image files themselves. He says there doesn't seem to be any other product that can import this format.
Is there any open source photo album software (preferably on Windows) that would be easy to modify to allow it to import this spreadsheet data? Or is there perhaps another solution I'm not thinking of?
Consider using the IPTC/XMP image metadata. If the files don't already have the right metadata, you might be able to add it from your spreadsheet using something like http://www.exiv2.org/
http://en.wikipedia.org/wiki/Comparison_of_photo_gallery_software (See IPTC support column)