do you mean you want to build a device that plugs into the USB port, and is operated (through software) via the computer's keyboard, to control the lights? This option would be relatively easy with a USB-serial adapter as mentioned, and possible (but not easy) with a PIC that can act as a USB device, such as the 18F4550. And if you don't actually NEED to be using USB at all, this would be even easier with the parallel port.
or do you mean you want to plug a USB keyboard directly into your device and have it control the lights? This isn't impossible, but for a hobbyist (especially one inexperienced with USB) it is PRACTICALLY impossible, the level of complexity is huge. If you need to do something like this, I would highly suggest you instead use a PS/2 keyboard, which isn't very hard to interface with a PIC (and I believe most USB keyboards will act like PS/2 keyboards if you use those USB-PS2 adapter plugs they usually come with)