Someones using my network?

Going from experience, it may very well be the Xbox. It might have accidentaly grabbed the PC's IP address. This usually happens though when someone is running a web server, and the router gets totally confused. Then again it could be someone else. Yea i can;t tell you exactly how to do it since i dont know what router you have but google the brand and model number of your router, plus the question "How do i set a pword on my XXXX Router?"