Thanks to all for your positive responses to my query.
It was actually my sharp-eyed wife who solved the issue.
I always open my files as TIFFs in CS4. For some reason, the format box defaulted to "all formats" without me noticing it.
Thus, when I opened a file, it was as "all formats" and this froze things up.
When K. noticed this and simply changed the format back to TIFF, all was well again, and TIFF seems to be "sticky", so all files that are now opened in CS4 automatically are opened as TIFFS--until it defaults again, if it does...
But then, I'll know how to deal with it.
Hurrah!!
Back to processing another shitload of files for our upcoming exhibition.
L.