Are you rendering JPEG images sequences, QuickTime movies with JPEG encoding, or what?
My first thought is that you may not have the right codecs installed on the network machines.
That would also be my guess. I ran into this kind of a problem at a client's office this morning. Ran an update on the network machine and everything works just fine now.
Perhaps you must just enable the Allow Encoding using Legacy CoDecs option in the Quicktime control panel on the network machine or else some of that may not show up. Of course make sure QT is installed correctly in teh first place, as Todd and Rick said. On a server, you may need to set special group policies for that to work, also.