Depending on how technical you are, you may want to try changing DNS settings to use Google's dns servers.
Might be a problem with your ISP's DNS.
Or your modem/router...

Could try changing your Internet Explorer settings to default. That helped people that had problems with the launcher, but I don't think that should affect anything after the launcher.

If changing dns servers don't help and your ISP refuses to help you, look into testing WTFast or similar just to see if that works. If it does work, it's most likely your ISP that is somehow blocking you.