Lethal voltages can still exist within the power supply,even after the power has been disconnected. The inside of the power supply is NOT designed to be user serviceable.
To clarify... when you open a power supply if you chose to do so at own risk... your just looking at the top of the PCB, inspecting capacitor tops. When opening a power supply removing the screws that hold the clam shell of the power supply together, you will end up with one half that has the PCB and other half is a lid. If really careful you can with power cord disconnected inspect it without sticking your fingers touching the internal parts on the power supply.
I have tested a few power supplies years ago to see if they stored any lethal voltages and the primary side electronics drain out quickly, and low voltage DC 12 volts or less drain out slowly. Power supplies have a drain on the line/primary side electronics as a safety to drain out high voltage. Its best to assume that there is no safety (drain) circuit, and be safe than to assume that the power supplies safety that will protect you.
Here is a schematic and
R1 is the drain on the primary side to drain out the high voltage capacitors when power cord is disconnected:
http://www.eleccircuit.com/wp-content/uploads/2008/08/200w-pc-power-supply-110v-220v-by-tl494.jpgIf you chose to inspect the power supply you have been warned that it could be dangerous. CH and members here not liable for any damages or death. Just like a gun I treat all electronics as if they are loaded ready to kill. If you do the same you should be fine. If in doubt about voltages present a multimeter can be used to test across capacitor legs before touching them. But if there is no need to touch them...DONT. Additionally power supplies are designed in most computers to have the component side exposed and the trace and dangerous component legs hidden on the bottom. So you would have to be very careless to get mixed up with a voltage discharge in a modern PC power supply. Additional PPE measures can be implemented such as handwear of dry leather gloves etc as well.
If your at all scared about looking into the power supply then dont.
Here is a picture of a common PC power supply. When opening the case this is what you would see, something very similar to this. as long as you dont go touching the inside parts or poking metal tools into the inside parts, you will be ok.
http://www.hardwareheaven.com/reviewimages/corsair-cx600m-psu/corsair-cx600m-psu_pcb-full.jpg* MAKE SURE POWER CORD IS DISCONNECTED FROM BACK OF POWER SUPPLY, NOT JUST UNPLUGGED FROM WALL IF YOU DECIDE TO INSPECT!
Failure to unplug the wrong plug can lead to death! But knowing the power cord to the power supply itself disconnected, you then know that the power supply is not being powered!
If you chose to open power supply, your doing it at own risk, and you fully assume all liabilities as a result there of. None of this is a statement that you must do this etc. Its just if you chose to do so, be very careful.