ok.... so it works on vista & XP...
have you tried uninstalling & reinstalling rivatuner..?? that might help...
or download a few more apps capable of fan control.....EVGA precision, galaxy EXTREME tuner... there may be a few more, but try these & see if anything will detect the fan..
also enter your device manager & make sure your drivers are all functioning & up to date.... there will be a small ''warning'' icon next to anything that needs attention..

SK