This is mainly a problem with Windows XP. Your laptop has automatically connected to the wireless signal and is using that instead of the wired connection. To stop this happening, disable your wireless connection. Most laptops have a switch or button on them to do this easily. Otherwise it can be done by right clicking on the wireless icon (In Control Panel -> Network Connections) and selecting disable.
NB: You will have to re-enable the wireless connection before you can use the wireless network again!