See if you can access it by its alternate (8.3) filename...
Bring up a command prompt window
Start->Run->"cmd"
Change to the directory that contains your file. For example:
cd "\documents and settings\username\my documents\files"
List the directory showing alternate names:
dir /x
In addition to the normal name, you should also see a 8.3 name for the same file that can also be used to access this file. It might look something like BOOKCO~1.EXT. Hopefully it won't have any illegal characters in it. At this point, try copying it to a file with a normal name
copy BOOKCO~1.EXT filecopy.ext
Then try to open it with your Photoshop application.
Fingers crossed...
JW