I can’t reproduce this error. We’ve tested it on Windows XP, and Windows 8.1, and it works fine. I don’t have a Windows 7 machine right now but I can’t imagine how it works differently on Windows 7 than XP and 8.1.
Are you running the osFirmwareUpdater.exe in the ‘windows’ subfolder, or are you compiling and running the source code in Processing? If you are running it in Processing, the working directory will be different and hence the relative path would not work.