Actimeter is described as 'fun little app that tells you how active you've been at your computer. It tracks and counts all your computer-related activities and events, and turns them into a single, easy-to-understand number from 0 to 100'. There are five alternatives to Actimeter for Windows. The best alternative is Venmon, which is free. Other great apps like Actimeter are Mousotron (Free), WinOMeter (Free), Taskbar Meters (Free) and TinyResMeter (Free).
This small utility measures your computer activity. It counts the keyboard / mouse presses and computes trajectory that you have arrived by your mouse. You can easily set it up to be automatically loaded on startup if you would like to have captured all your activity.
TinyResMeter is a small tool that displays useful information about system resources in real time, but it also gives much more detail when you right-click on the displayed fields ! Why TinyResMeter ? - It is written 90% at pure API level to keep it as small as possible.