Did the shocking just start or has it been happening all along?
It is not the marble top. As stated marble is a poor conductor.
It is your PC.
The current, having nowhere else to go, left your PC, traveled through the USB cable (or perhaps the bare metal on the case) , through you, and then to ground (earth).
That being the floor and you got shocked.
Somewhere in your computer there is an electrical short that would normally go to ground through a designed path.
Your electrical system does not have a built in ground. So the electricity, which desires very much to go to ground, finds its way through you.
You need two things: 1) a qualified electrician to check out your home's electrical system and establish some grounding per local codes and requirements, and 2) a knowledgeable person to look at and test your PC.