Never, never plug in a keyboard into a computer that is already running! On most PCs, the keyboard connector is directly connected to the motherboard. Plugging a keyboard into the PS/2 slot when the PC is running can send an electrical shock through the system, potentially damaging the motherboard and frying the components therein. And I think we all agree, this is bad.

You should not plug in or unplug any components from your PC while it is running (unless, of course, it is USB which are designed to be hot-swappable). If you need to plug in a new keyboard, shut down your PC first.