I have just inherited a network at my work- 1 primary server and about 65 computers running XP. All machines have Gigabit ethernet cards and all are connected through Gigabit switches and cat. 6 cables. The workstations send large numbers of image files to the server where they are held until they need to go to a printer. And I'm talking Gigs of files. We get a network slowdown when large number of files are coming from the workstations at the same time large numbers are also going the the printers. 
All machines have to be on the same subnet because of some proprietary software.
Is there anything I can do to lessen this logjam? 
Any help would be greatly appreciated.