Windows XP Professional
I received a message saying that the network cable is unplugged; I check the plug and it is in the right spot and it is in securely. I tried rebooting the computer, and that didn't help, so I tried the computer with a Live CD of Ubuntu and I had an internet connection with that. I ruled out any issues with the hardware, so there must be something wrong inside of Windows and I can't figure it out.
EDIT: I also went through a bunch of troubleshooting, with no luck. There haven't been any problems with internet connection, and I've had for a year. The I only thing that I recently changed on the computer was the monitor.