You have to look deeper than total wattage!
Power Supplies have to supply power at several voltages, and the wattage available at each voltage has to be above PEAK actual needs by at least 20% for stable operation. Most modern ATX Power Supplies need to provide +12V, +5V, +3.3, -12V and perhaps -5V as well. Each of these may be provided by one or more "Rails" and if the rated amperage of any rail is exceeded the voltage will drop. So your 400W power supply may provide plenty of power on the 12V rail, but fall short on the +5 or +3.3 V rails.
Things like fans and hard drives may pull 2 or 3 times more than their running amperage during startup, if that is exceeded the systems gets low voltage for that power rail and either shuts down or behaves erratically.
A good example of this is a system that was recently brought to me which would not start the first time you turned it on, You would get a BIOS beep code for Video Failure. The problem was a power supply, that did not provide quite enough power on the +5V rail. By turning the system on, then after the BIOS error resetting the system, it would start normally.
The point is, you need to use a power supply that is approved for your processor / motherboard combination, or you need to have a complete understanding of power supply in order to choose a power supply that will work properly with your system.
I would not use a shop vac the combination of materials and design could well result in static discharge. Canned air is your best bet. Also not a good idea to blow 'right through the fan (which will whirr). This can cause dirt to be blown into the bearings of cheap fans and shorten their life.
