Vista - The disk is write-protected. Remove the write-protection or...

I am migrating from an older PC to a new Vista 64-bit Ultimate PC. In the
past, the way I circumvented all the sharing issues of copying files from my
old PC was to make an Acronis True Image 10 backup of the old PC. Then I'd
mount that backup image on my new PC and copy files from that mounted image.
It worked fine in XP but in Vista I get a "The disk is write-protected.
Remove the write-protection or use another disk" error message on every file
I try to copy. It doesnt matter if the archive is local or on a network HD.
It doesn't matter if the file originates in a shared directory in the mounted
image. And it doesnt matter if I try to copy to a 'public' folder on the
destination.

Help, please?

Craig