From another thread:
open rivatuner
click on the power user tab
read, then ok the warning that comes up asking if you are experienced enuff to use it
then look at the list displayed, find the one that says RivaTuner 2.0RC 15.2\NVidia\Overclocking
make sure its the one with Nvidia, there is another listing without Nvidia, just saying Overclocking
then click on the plus sign to open a menu under this.....
you should see probably 2nd item down is DisableInternalClockTest
that is written on a light grey background, look to the left, u can just make out a greyed out tickmark,
click that so its a solid colour
then look to the right of the DisableInteralClockTest writing and see the dark grey panel, type a 1 in there
then apply and ok whatever it says..
then you should reboot your computer
when thats done open rivatuner again
where it says on the Main tab Driver Settings, you should have Forceware Detected in the box.
next to where it says customize is a little box with an arrow, click that to open some options
chooser system settings
first tab should say Overclocking
check the Enable driver-level Hardware overclocking box, this then allows you to change things.