To start with, close everything except Explorer & Systray. If you`re on the internet, don`t close down your firewall and antivirus. Use this as a start point and open only the programs you need, then the PC will be running on maximum available power as long as you`re on it.
if you need to open another application, make sure you close it using the top LEFT icon in the window, then hit close in the window. Just clicking the `X` doesn`t close the complete program, and some of it can still be running in the background.