Does your system have a power cord with a power brick on it somewhere between the wall plug and the back of the PC, or it is just a normal cord from one end to the other?
What I mean is, just like on laptops, than have the power "brick" which is the transformer and adapter. Does yours have that or do you in fact have an actual power supply inside the PC?
If you have a power supply inside the PC, then on one of the sides there will be a specifications decal that looks something like this. In SOME cases, it may be necessary to remove the four screws holding the PSU to the case, and turn the PSU about halfway in order to see the decal or label. It should be obvious that you will want to shut down the system first and then flip the switch on the back of the power supply, off. Then unplug the cable from the wall or PSU, before unbolting the PSU from the case.