I am not a tech geek so all this methods are created by other tech geeks on the net. Follow these methods at your own risk.
1. Go to Powermizer Manager, download the program and follow the instructions. Since I am using a laptop I could not get this to work even after I install the official mobile driver from Nvidia.
There are adequate support here do browse through the FAQ or send the author your report if the Powermizer manager does not work for you.
2. Go to Dox's Customized Forceware Release. This is actually a forum page from laptopvideo2go forum. Dox's Customized Forceware is very good. I have used it a few times and it provides better performance. Dox strips off useless stuff to make Nvidia's driver better. If you have been using laptopvideo2go drivers and inf, you will know you have to go through a few steps to install the drivers. Dox's drivers is easier to install and the news ones does support disabling Nvidia Powermizer. Feel free to post your problems and Dox might try to help.
3. Go to http://cpu.rightmark.org/download.shtml and download Rightmark CPU clock utility. After you get the program to work, just go to Advanced CPU settings and uncheck all entry except Enable C5 and C6 modes in the Chipset section. You need to check the Apply these settings at start up for the program to remember your desired settings. This method involves manually disabling Nvidia Powermizer. First you need to trigger the effect. Open a few video heavy application and you will see the performance drop. This happens all the time because I live in Asia and it is hotter. Go to the Monitoring page of Rightmark CPU and you will notice the CPU core clock and throttle has gone down to half. Go to Advanced CPU settings, fiddle with one setting but change it back and you will see the grayed out apply button is now press able. Click on it, go back to the monitoring page and you will see the throttle went back to full speed. This effect will last you the whole day unless you restart your PC. I recommend this method only if the above two does not work. This method still works for me at the moment.
There are other steps, ones that involves registry setting, I have fiddled with them and because I am not a tech geek I did not manage to get it working. So try out these 3 steps and if you still can't disable Nvidia Powermizer you will have to find another way.