Lose Internet connection on wireless network

I have wireless network setup in my home that works fine. We have an HP desktop machine running Vista Home Premium connected directly to the Zyxel router and an HP laptop with Windows XP Pro connecting wireless. The problem occurs when the laptop connects to the Internet; the desktop cannot connect to the Internet. The laptop can always connect to the Internet no matter what. As soon as the laptop disconnects from the Internet, we are able to access the Internet with the desktop again. I know it has to be a setting somewhere but I don't know what to look for.