Windows 8 / 8.1, and possibly 7, will auto-launch a web browser on connection
to a wifi network with a captive portal. This is a security risk -- anyone
could set up a wifi network with malicious script in the capture page, and
windows would automatically navigate to it without user intervention. Or, on a
public wifi network with such a portal (such as an airport), malicious code
could be injected in transit by an intercepting access point.

To turn this behaviour off:

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

set 'EnableActiveProbing' to 0

(the default value is 1)

Then restart the computer.

According to http://technet.microsoft.com/en-us/library/cc766017(WS.10).aspx
"Setting the value to 0 prevents NCSI from connecting to a site on the Internet
during checks for connectivity."

See also http://blog.superuser.com/2011/05/16/windows-7-network-awareness/