As rolli59 said "AMD Cool and Quit" & "Intel Speed Step"; I'm using a intel core2duo E6400 clock speed is 2.13GHz, and on CPU-z it says that my processor clock speed is 1.6GHz when doing no heavy processing.
Lets just say I run my AV, the processor multiplier will change from x6 to x8 and my clock speed will change to 2.13GHz. When it's finished doing what its doing then it goes back to x6 1.6GHz.
It's all about energy saving.
So try and do something that requires a bit of CPU usage and keep a eye on your multiplier and clock speed, and see if it changes.