Go into the router's wireless security settings and change the WEP/WPA encryption key. Then delete the existing 'profiles' in the wireless settings of the PCs and go through the procedure to "Connect" again. You should be prompted to enter the WEP/WPA key and can now use the new key.
(Assuming Windows XP) - If using Windows "Wireless Zero Configuration" to manage your wireless networking, right click the Wireless Network Connection in the Network Connections folder, select Properties and then the Wireless Networks tab. Highlight the existing entry for your network and click the Remove button click here