windows 7 - How to automatically shorten names in a folder and its subfolders?

I'm trying to do a backup on my Dropbox folder but I get the following message:

Source Path Too Long

The source file name(s) are larger than is supported by the file system. Try moving to a location which has a shorter path name, or try renaming to shorter name(s) before attempting this operation.

My Dropbox folder contains 115752 files, so I wonder if there's a way to automatically shorten the names that are too long?

I'm running on Windows 7.