So, how many USB ports does the PC have? If you put another USB device in the same port, does it work? The reason why, is because Windows 7 can pretty much self heal, if it is on the internet. With that being said, you may need to delete the drivers, and reboot. Then plug the headset in. Once again, make certain that the USB is still good, first.
Take it easy there. If other devices work in the USB, but the headphones don't, then they might have reached the end of their usefulness. What is the model of headphones so I can look at the specs and try to determine if there might be a way to troubleshoot the headphones themselves.