I can no longer open Powerpoint or Excel files on my Mac.

I can no longer open Powerpoint or Excel files on my Mac. I can see the files in finder but when I try to open one I get this error msg: "Powerpoint cannot open the type of file represented by <file path><filename>.ppt (or .pptx)" Same thing with Excel. When I try to open the file from within the application, I get this error message: "Powerpoint cannot access <filepath><filename>.pptx because the file has been moved or deleted." (And, no, it hasn't been moved or deleted.) I get that same error message when I try to create a file in the application and save it.

Now, I have sent some of these "afflicted" files to my wife's Mac via our dropbox, and she can open them with no problem. (She has the same OS and software as me). So the files themselves are not corrupted. This problem surfaced recently for no apparent reason. I hadn't just updated anything. I have since trashed my Microsoft Office folder (with all the Office apps) and re-installed. Same problem. I am using Microsoft Office 2008, with OSX 10/10.4 on a MacBook Pro. So far no problems with my Word files (thank god!).