I have had nothing but trouble with the X83 since we got it. I am using Panther now and it seemed to work fine (direct USB) when I first installed but now is not printing anything. I think the 'helper application' is getting confused with the multiple user login.
What I am not sure about is how the windows printing system deals with different print formats. Have you got the Lexmark drivers installed on the Mac? Could it be that the file the Windows machine is getting is in the wrong format or even just completely empty? Perhaps you could stop the queue on the windows machine, track down the incoming file and see what is in it?
I'm sorry I can't be more helpful - all I know is it always takes me a long time to get the X83 settled whenever I change anything!
James