Hello,
I recently had the same dilemma with another fan. I'm not sure of the details of how your fan connects to the PC, but what I did was to simply connect my fan between the red and black wires coming from the PC power supply rather than the yellow and black, so it ran at 5v instead of 12v.
Other solutions suggested to me included putting a resistor in line with the yellow wire, to reduce the voltage and hence the speed, or even a potentiometer so I could adjust the speed. Hope this is some help.