Slow network printing

We are currently using Deskjet 6940 printers over a network and are having
issues with the time it takes for documents to print.
We are using Windows Server 2003 and a variety of machines using either XP
or Vista. When sending documents to print it can take up to 5 minutes to
start printing but when printing directly via USB they print almost
instantly. The users are getting increasingly frustrated with the delay and
we have so far been unable to find a solution to this problem.
The strange thing is that when printing from the machines within the IT
department documents print out almost immediately to any of the printers
within the company.
All are using the same drivers and they are set to print right away. We have
a laser printer on the network and this has the same problem.

The other problem occurring as well is they won't print jpegs. The printer
in our office will but the exact same printer elsewhere won't. The file goes
into the print queue but disappears and never prints. Have tried
uninstalling then reinstalling and creating new ports, nothing seems to
work.