Cannot connect to iTunes Store

When I try to sign on to the iTunes store, I get this message,

"iTunes could not connect to the iTunes store. The network connection was refused. Make sure your network settings are correct and your network connection is active and try again."

My network settings are correct. I ran network diagnostics and my internet connection is working normally. (In fact, I'm on the internet right now, correct?) I have the latest version of iTunes. I used Keychain First Aid and it found no problems. I tried unplugging my cable modem and wireless router, then replugging them and restarting my computer.

I sent an email to apple Support and they wrote back and told me to do the things I've already done. Am I missing something?
Another thing...when I tried to install an apple software update, I got this message: "Software update can't connect to server" I don't understand this. Thanks for any help you can give me.