I think he wants to know before he buys it ...
What you mean is, that your airport express is being used as a relay station, in which it's simply extending the wireless internet range that is being "created" by a main router, correct?
If so, and if your ethernet port on your Airport Express isn't being occupied by some other line (since the airport express only has one ethernet port in all), then you can do this. Your PC will just act like that ethernet connection is just a standard ethernet connection. The only thing you will have to do to enable the ethernet connection for any use, is to open Airport Admin Utility (AAU), and select the Airport Express' options. Then look for an option in which reads something along the lines of "enable ethernet port usage".
The only thing I'm unsure of is that whether you can open AAU on a PC. You'll need to call Apple or ask around to see if it's compatible on PC's, because if not you'll need to use someone else's Mac to open AAU and enable that option.
Good luck