Labtop Cannot Get Wireless Signal

I am using a Belkin 802.11g wireless router. Several days ago, my Dell labtop cannot connect to internet through the wireless router. It says the connectivity is very weak. I can see the other wireless networks near my home. Can anybody tell me that it is the problem of the router or the wireless network adaptor in my labtop? I still can connect to network through network cable on the router. Thanks.