Windows 2003 R2 Server Disk Trashing

I have a Windows 2003 R2 Server functioning as a Active Directory and file server. It is disk thrashing all the time when users (60-70) start opening documents over the network. The server 2 X Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz has 4GB of memory, average memory avialable at any given point in time is 3,000mb. CPU average is 17%.

Total Memory 4096MB
Free Memory 3067MB
Max Swap File 12132MB
Free Swap File 11089MB

Page file is not located on the disk where thrashing occurs.

The disk subsystem: 4 X 500GB

Any ideas???

Thanks for the help