It doesn't reduce the CPU usage of a process, it changes that priority of the process. The way to test it is to bring up your task manager, right click the process in question, point to 'Set Priority' and see what is checked. If it's on low, then Process Tamer is doing what it's supposed to be doing. The objective of the program is not to reduce CPU usage, but to make it so windows won't become unresponsive while a program is using a great deal of processing power. That said, it works fine on my Win2k machine.