Sharing WiFi to ethernet troubles

I built a new Windows 7 PC recently, built specifically for gaming, but I forgot to include a wireless card in it on a fluke. While I wait for the opportunity to get one, I'm trying to use my Mac to give it internet since my house doesn't have any ethernet ports.

I thought I could use the Sharing ability under System Preferences to share the wifi connection on my Mac to my PC, via an ethernet cable. It was working at first; the PC was picking up the internet alright, even if it was a bit slow. But after a day or so, the connection stopped working altogether. The cable is plugged in properly, but the PC can't connect to the internet regardless. When I run the problem-solver on Windows, it tells me to unplug my router or simply that the connection is broken. Among other things, it's said that " "Local Area Connection" doesn't have a valid IP configuration " as well.

Can anyone point me to the source of the problem? It likely isn't the PC. Am I doing something wrong on my mac? Could this possibly be an issue with my service provider? Thanks very much.