I'll assume your SSID has been changed from the default to one of your own.
click on Start> Control Panel> Network Connections.
Right mouse click on Wireless Network Connections and select properties.
Click on the Wireless Networks tab.
In the preferred networks box, remove all Networks/SSIDs which are not your own.
Highlight the only remaining Network which is your own and click on the Properties button.
Select the Connection tab and make sure the "Connect when this network is in range" is ticked.
If you ensure that your network is the only, or at least the first one in the list of preferred networks (use Move Up / Move Down buttons), yours will be connected to automatically as the 1st default.