It would be safer for you computer if you leave the power cord plugged in. That will let the static discharges go to ground instead of building up potential in your PC. Just make sure you turn off the main power switch that is on the power supply, or if you don't have one of those, the power bar. After that is off, press the power button on the front of the PC to discharge the capacitors in the power supply. LEDs will probably blink, and then that is it, no more power and a safe, grounded computer to work on.
And just a tid bit of info on ESD (Electro-Static Discharge), circuits can be damaged by 100V or even smaller. Humans cannot feel a shock less than approximately 3000V.