Windows 7 only: If you're not into Windows desktop gadgets, there is another way to easily monitor your computer's CPU and RAM: Taskbar Meters puts simple, attractive resource monitors right on your Windows 7 Taskbar.
Taskbar Meters contains two programs that you can pin to your taskbar — one that monitors CPU, one that monitors RAM. Each has a coloured bar that shows how much of that resource you are using, complete with colour coding. In the program's preferences, you can decide how often to update the meter and what percentage of usage constitutes "yellow" and "red" colours on the taskbar. Each icon also has a jumplist that lets you easily open up Task Manager or Resource Monitor to get a more detailed report.
Taskbar Meters is a free download, Windows 7 only.