For Windows 7
Go to:
'Start'
'Control Panel' (if you cannot locate this, just search "control panel" it will come up)
Under Control Panel, go to:
'Network and Internet'
'View network status and tasks'
'Manage Wireless Networks
At this point, you'll see all the networks your computer is picking up wirelessly. To "DELETE" networks that are unwanted or interfering with your "Preferred" network, Select the one you wish to delete by one left hand click. This will highlight the network. Then, right hand click, "delete" will be one of the small drop-down menus. Repeat this for any network you want gone. Once finished, just close, all new settings (i.e., deleted networks will no longer appear) will be saved.