Please tell me the entire text of the error message you're receiving.
That error code indicates a connection failure at the most rudimentary level. It's possible you tried it while the server on the other end was down, although I'm not aware of any significant downtime in the past couple of days. Something is preventing the Launcher from connecting to the Internet via port 80. It's likely you have a software firewall that is blocking the Launcher (Windows XP has one built in, you might not even be aware of it), in which case you will need to tell the firewall to allow the Launcher access.