Check the literature which came with the router for how to access the router's setup screens via your browser with an ethernet cable between computer and router.
Once in, review or renew the wireless security type (WEP/WPA) and passphrase. Also make sure wireless access by list is disabled.
Then apply wireless security type and passphrase to the laptop's wireless adapter interface (or Windows Wireless Connection) and try removing the ethernet cable and see if you can connect wirelessly.
If you have problems, a useful tip is to disable wireless security in the router temporarily and try to connect -- this helps narrow down causes of problem.