hi, this was really getting frustrating, i tried installing the drivers through Hardware Tools  ad also tried  Envyng  but nothing was working ! finally i got the solution , manual labour is sometimes more fruitful!  . I downloaded the driver .run file from the nvedia site, switched over to command mode ( Alt+Ctrl+F1 ) ,stopped the X server ,by writing :
 sudo /etc/init.d/gdm stop  and then installed the driver , after that i started the X server
 and was at last successful 
 Hope this was useful!