First of all, latest Nvidia driver is 40.72, not 40.78.
The Coolbits link I gave before is for the 40.xx version (officially from Nvidia - 40.41, 40.71, 40.72 - all betas). I'm using the 40.72 revision under XP with the mentioned coolbits, and everything works fine. Only 40.72 is available for Win9x/ME, too.
Which Windows are you using? What graphic card do you have?
Basically, try to reinstall the driver (first do an uninstall from Control Panel - Add/Remove Programs, if using Win9x/ME, you can use
Detonator Destroyer - as there is a chance that some crap from the previous driver installation is in your system). After that, unzip the coolbits for
40.xx revision of Detonators, right click on the file and choose
Merge. After you get a message on the screen where you are asked if you wanna merge the settings from the key into the registry, click on OK and it should be done. Now you are able to open the driver settings through desktop properties, advanced... After all, there should now be the possibility to change Clock Frequencies. First you have to choose "allow clock frequency adjustments". After that you probably will have to restart the PC. Then go back into display properties and clock frequency settings, choose the desired setting for memory and GPU speed, click on test settings - if this goes through you will be able to press apply and you overclocked your baby.
Hope this helps.