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:


set 'EnableActiveProbing' to 0

(the default value is 1)

Then restart the computer.

According to
"Setting the value to 0 prevents NCSI from connecting to a site on the Internet
during checks for connectivity."

See also