The issue your experiencing usually occurs when you delete the Zune Software, switch computers, reformat, install a new operating system, etc.
It considers all the content on your Zune to be "from another computer" because the current installation of Zune didn't put the Music and files on the device. This shouldn't hinder the usage of the device but to correct it erase all data and resync.
Jared Marino
Administrator
Zunited Zune Community and Technical Support
http://www.zunited.net/