Some of those will hold the charge on the capacitors for days, you cannot predict.
Get someone who is familiar with electronics to open it for you, and short out the High Voltage capacitors. After that, it would be safe. (there are usually two of them, sometimes 4)
I use an insulated screwdriver to short them out after opening. One option is to place the supply face down (circuit board down) on a large piece of aluminum foil on some soft cloth, and press down. Listen for any "spark".
Still, it is safest to find someone who knows what to look for, and clear the charge - try any electronics hobbiest, amateur radio person, electrician, etc.
Unplugging the power and turning on the computer to "drain" it may not work. These are switching power supplies, and some want to sense the input voltage before they start their "switching" cycles. If the voltage is just 120V instead of 180V on the capacitors, it may not start up, leaving the 120V on the capacitors.