It is not only Win7 ... I have the same problem with XP ....
But NOT on all jpgs ...
Seemingly only those that have been "handled" by photoshop on a MAC ... but NOT ALL photoshop on a Mac photos.
I am not an idiot ... I and my wife have been using MS Picture Manager to quickly re-size pix on our PCs for YEARS. We handle THOUSANDS of photos per year. Something happened within the last 4 months or so that has created this "can't save" problem.
We can EASILY and routinely "resize" our digital camera pix, no problem -- so we KNOW HOW to do it.
We can do SOME other jpgs ... but not ALL other jpgs. Opening is NOT the problem. Sizing is not the problem ... the problem is SAVE AS xxxxx. And voila, the error message appears: "some files cannot be saved. please try again"
It is NOT a 'read only' problem ... (SaveAs should take care of that)
It is NOT a "cmyk" problem ... it happens with pix that have been handled in Photoshop/Mac both RGB or CMYK
I had the same issues and it was an easy fix. You just have to run the program as an Admin ( search for the app and right click and "run as admin"). works like a charm