CpuUsage

From Outertech:

Have you ever faced a situation where you have waited for a program to complete and had the feeling the program may have crashed? Wouldn't it be nice if you could always see if your CPU is really working? Well, you could have the task manager window in front, but how about a program that doesn't waste valuable screen space? CPUUsage System Service is a free tool that displays the cpu load as a tray icon image. It's very easy to use and utilizes minimal resources.

CpuUsage installs itself as a system service and loads automatically when Windows starts. If you want to use it only when required, you can disable the loading in the tray icon menu