I'm staying at a friend's place and can only access the internet when I'm in the same room as the router.
When out of the room my computer detects the network at two bars of strength but after a short time denies me access, when I run the network troubleshooter it says that it cannot detect the proxy settings.
My friend can connect fine to the network from anywhere in the house.
Both of us are running Dell Laptops with Windows 7.
My card is a Dell Wireless 1510.
The router uses WPA2 encryption.
Maybe your wireless adapter is tuned to receive on a particular channel and you just cling on to a feint signal whilst near enough to the router. Either look in the router's settings for the channel number it uses or try to set your adapter to a roaming position.