Very slow networked PCs -memory problem?

Our PC’s on a small office network run very slowly, even with only 1 application running. If I open task manager, the memory usage seems to be very high, even when CPU usage is very low.
Our PCs run Win2000 with NT4 on the server.
After restarting the PC, with no applications running, the CPU usage is 2-3% but the memory usage is around 116600K of 215784K. There are 29 processes listed, but their memory usage doesn’t add up to anywhere near the total.
Is this normal for Win2000? I can add more memory to the PCs, but there must be some other problem??

any help greatly apreciated!!
Lee