With the power supply OFF and unplugged, stick a pencil (or whatever will fit) into the fan grill and see if you can spin the fan. If it is very stiff or does not move at all, then you need to either replace the fan, or replace the power supply.
To replace the fan, you'll need to take the cover off the PSU (which voids the warranty). The fan will be connected one of two ways: either soldered directly to the PCB, or have a 2 or 3 pin connector. Remember that a power supply can hold a dangerous charge inside for a long time, even if unplugged.
Intel Q9400 | Gigabyte EP45-UD3P | G.Skill 2x2GB DDR2 1000 | XFX HD5870
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