How to make my laptop use full GHz?

I have a 2.1GHz double 2 due core processor and i want my laptop to use the full 2.1GHz it keeps going up and down so when i play a game sometimes it starts to lag and sometimes smooth i hate that no matter how much i close programs or set the game on low settings it still does that i have a GMA 4500DHM with 128MB of VRAM wich i know is preatty bad but when i upgraded my laptop from windows vista to windows 7 my laptop was using the full 2.1GHz and games never lagged but when i defraged my hard disk my processor started to go up and down again what can i do to make it go full power again??? oh and by the way i cant go to my BIOS it has a pword and i dont know it so if there is a way i can do it in windows 7 please tell me {:0)