Assuming you have a NVIDIA graphics chip and that the driver is correctly installed, you should be able to access the control panel directly from the main Windows desktop by right-clicking an empty space.
If you're running Windows XP, Windows Vista or Windows 7, it is also accessible via the Start Menu.
If you don't already have the appropriate graphics drivers installed which include the control panel, you can obtain it by clicking the "Free Download" link above. Be reminded that if you don't have a NVIDIA graphics chip installed (IE. An ATI/AMD graphics chip or an Intel HD graphics chip), then you will not be able to run the software because it is designed for use with NVIDIA chips only.
Provided you run the NVIDIA Display Control Panel as an administrator, the changes should remain in effect until you change them. Keep in mind that when a guest user or a user without sufficient privileges attempts to modify the settings, they may not remain in effect the next time the user logs in or the system is restarted.
Depending on your system configuration and the brand of your PC (if any), you can try changing the overclocking settings of your graphics display adapter via the system BIOS. If applicable, you should be able to unlock different settings depending on the BIOS version (firmware) you have installed.
Be reminded that some manufacturers (such as Sony or Dell) don't normally allow you to overclock or modify certain settings, leaving the BIOS options pretty boring and lacklustre.
Unfortunately, there are no straight answers to this question since each configuration and setup vary from each other. There are several different settings available depending upon your graphics chip and "playing around" with them and experimenting are normally the best ways to optimize your graphics adapter for best performance. Furthermore, many games produce different results depending upon the graphics setup. Luckily, NVIDIA Control Panel can modify the settings depending upon which application or game you are running, thus allowing you to apply settings based on the performance of the game.