If two different keyboards fail to work, it sounds more like a dead port. So, assuming it's a PS/2 keyboard (small round plug), you can either buy a new USB keyboard, or a PS/2-->USB adapter. Personally, I'd go for the new keyboard, because I've seen plenty of those adapters literally come apart at the seams and become useless.
If you've got one of those obnoxious Dell systems without any PS/2 ports, then try moving the keyboard to a different USB port. If you have any front ports, those are often times on a different USB controller from the back ones. You could probably also buy a USB hub if you're running short on ports.
My PC cann't see the keyboard althought i go to add new hardware.I thought this keyboard is broken, then i changed a new one, but it didn't work. Maybe its drive is deleted. How can i do with this situation? Please help me . Thanks