Before thinking big, like clutch or gearbox, be sure all the hydraulics check out. Make sure the slave is not leaking, that there is enough fluid in the system (shares fluid with the brakes), and that the slave is fully extending when someone depresses the pedal. You can easily see the slave by looking down from the top just to the right of the throttle body/IAC. Look for a rubber accordian boot. If there is any fluid on or underneath it, it's leaking and chances are it isn't holding the pressure necessary to disengage the clutch. If you need to replace the slave, also replace the master and thoroughly flush the system. Debris from a failing master can be stuck in the line and destroy a new slave.