Really, what you need to do is to carefully look at what is making it slow. The system monitor is a simple way to observe what the machine is doing, and more particularly, what various processes are waiting on.
It is axiomatic that the first thing you should do is to add absolutely as much RAM to the machine as it will hold. "Chips are cheap" now.
If your I/O subsystem can support it efficiently, add more disk drives and spread things around on them.
Stop any un-necessary services.
Turn off fancy graphic screen-savers.
Turn off virus-scanners. Instead, set up a system of user-IDs and privileges so that "any Tom, Dick, or Harry's web-page" won't be able to tamper with critical files in the first place.