In a command prompt (Be an administrator, don't be a boob. If you insist on using that stupid User Account Control, then right click on your Command Prompt shortcut and run as Administrator)
bcdedit -set loadoptions DDISABLE_INTEGRITY_CHECKS
Since it's a boot loader option, it obviously doesn't take effect until you reboot.
This allows RivaTuner to install. It only applies to 64 bit Windows, because 32 bit isn't as stringent (this step won't be necessary in 32 bit Windows7 or Vista unless you want to)
As a "bonus" (quotes because I think it's retarded to not allow you to force it if desired) you won't be bothered about unsigned drivers again either.
Now, applicable to current RivaTuner with current Nvidia drivers in any Windows OS (XP, Vista32, Windows 7 etc.), Nvidia has changed the way it reports the driver version. RivaTuner 2.24 doesn't know what to make of it so it just says that it's an unsupported driver, and the settings are unavailable.
This workaround will be necessary until RivaTuner 2.25 is released, sometime in June (according to Unwinder)
In RivaTuner, go to the Power User tab. Accept the message that it's for power users (duh).
Make sure that the current database is RivaTuner.rtd (it should be by default). If not, browse for it using the Open Database button.
At the very top, the first registry key is called RivaTuner \ System
Expand that and look for a value named ForceDriverVersion
Make sure that RivaTuner is not in hexadecimal mode (It probably is by default... it is for me). Click the button below (hover to see which one it is) to toggle that off.
Double click the ForceDriverVersion value and enter the driver version with no decimal place. For example, if your driver is 185.81, enter 18581
Click OK to exit RivaTuner and start it again. It will still say "No supported drivers detected for this display adapter" in the field under Driver Settings , but beside it will be the triangle Customize button and the settings categories are available and working.
Hardware Monitoring (Target Adapter, Customize button, Hardware Monitoring category) works too once you make the ForceDriverVersion change.