If the PSU is past warranty, you can also open it up and blow the dust out.
Remember, that there is either 110v or 240v in the PSU on the primary side.
That's enough voltage (and amperage), to use dust as a conductor.
Too much dust in the PSU can kill other components, and can catch on fire as well.
If the PSU "only" overheats and dies... you'll be... lucky.
If you do decide to open the PSU, remember to discharge the capacitors first...
1. Turn off the computer and turn off the PSU, but leave it hooked to the wall outlet.
2. Hold the power switch on the case down for about 20 seconds. This will discharge the capacitors.
3. Then, unplug the PSU from the wall out let and open it up.
4. I still wouldn't touch anything in there.... just blow it out.
P.S. These are U.S. instructions. The EU does weird things with electricity.

(
@Try3)