There's some kind of bizarre behavior from Windows 7 linked to the way that it manages locations for security (home network vs. business network vs. public network). A fix that works for some is to disable this feature through modifying a registry key.

1) Open RegEdit from the Start menu (type regedit.exe into the search window)

2) locate 

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NlaSvc\Parameters\Internet

3) Under the Internet key, double-click EnableActiveProbing, and then in Value data, type: 0. The default for this value is 1. Setting the value to 0 prevents NCSI from connecting to a site on the Internet during checks for connectivity.

Hope this helps!