A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
I don't know if it's true of the other Office apps, but at least in some cases, the root of the problem was in the temporary file that PowerPoint opens in the same folder as the file you've opened. Things would get dicey when you opened a PPT file in a synched Dropbox folder. PPT creates the temp file, Dropbox goes "Oooh. New file. Must synchronize it. Now!" and puts a lock on the file. PPT tries to write to the temp file, finds it locked and can't cope.
I'd try opening files using the other Office programs while watching the folder the file's in (after having made sure that your options allow you to see all files ... if you can't see PPT opening a temp file, this won't work for other apps). If, say, Word also creates a temp file in the same folder, it may be subject to the same bug.
Personally, I'd probably move any files out of synched Dropbox folders (or external drives, thumbdrives, anything) and onto the local hard drive or at least an independent network folder before editing, then put them back when done. That way if *anything* goes wrong, you've only lost the local copy and can still revert to the original if need be.