Seeing as it is a Dell the option may not be there, and they don't show it as an option here,
http://support.dell.com/support/edoc....htm#wp1109861
but have a look in the bios (should be in the Power Management section) for anything to do with fan speed/control, may be listed as PWM. If it is there enable it and the CPU fan at least should run at a lower speed, and only speed up when there is a load on the CPU and the extra rpm's are needed for extra cooling.
If it is the video card you want to slow down you can try RivaTuner,
RivaTuner v2.24 download from Guru3D.com
If you do try it just play around with the fan speeds. Here's a guide on how to set them up (both manual and automatic fan control),
RivaTuner 2.0 fan speed & Overclock Guide