Spence .. some of the old photo's are just not high enough resolution to zoom in and get any clarity . I see that a lot .
I use ACDsee with some success but unless you have something to start with you'll be up against it .
If you downloaded the pics from a website make sure you downloaded the largest file size .Some of the archive websites only show the smaller file size , but sometimes have a link to a larger (clearer ) file size .
Cheers .. Chris