I would check all the network settings. Have you set the printer to be shared
on the desktop computer? Are you using the Add Printer wizard in Vista? Are you using "Search for a printer"? Instead, use the option to input the computer name of the desktop computer with the the printername, eg. \\shardonairCompXP\printername. Another is to use the IP
address of the desktop computer, eg, \\192.168.0.1\printername.
Look at the other options available in the Add Printer wizard.