Network running super slow!

I have around 250 computers on my network. Windows 95.98.NT.2000 and XP OS.

Servers are mostly NT and some 2000.

Recently, within the last few months my network seems to be crawling. Files are taking an extra long time to transfer. Sometimes email takes a long time to open and networks applications are almost impossible to use. I need info on software and or anything else that can help me determine what the problem areas are.