It sounds like you're either using one of these
http://www.yescustomer.com/image_manager/attributes/image/image_47/1097252419_4240099461_full.jpg
(which lets you plug an old-fashioned PS/2 keyboard into a USB port)
or one of these
http://upload.wikimedia.org/wikipedia/commons/5/57/USB_to_PS2_mouse_adapter.jpg
(which lets you plug a modern USB keyboard into an old-fashioned PS/2 socket).
If it's the former (and your keyboard isn't recognised by Windows) you might not be able to use your keyboard unless there's actually a PS/2 port on your new computer. (Many new computers no longer have them but some still do). If so, simply remove the adapter and plug your keyboard into the PS/2 port. (It's probably coloured purple).
If it's the latter then all you should need to do is to ditch the adapter and plug the keyboard into any available USB port.