Have you enabled cpu q fan control in bios? The k2 fans are rated as follows,
140mm, 1100 rpm
120mm, 1500 rpm.
Under monitoring in bios, enable cpu fan control and select one of the profiles, silent, standard and turbo iirc. Save theese settings and boot into windows. Also, what connectors have you connected the fans too on the motherboard? The fans should ramp upto full speed under cpu load.