Your work computer may have some weird permissions that are getting in the way. Possibly to prevent people from transferring data from the PC to a USB drive. It could also be automatically encrypting the file on the PC and the Kindle doesn't know what to do with the encryption on the file.
A "green key" is not an icon or symbol that I've seen on any of my Windows files, so I suspect it may be third-party security software installed by your paranoid SysAdmin. Do you have any friends in the IT department?
