Do you have a firewall/router? Do you connect through a proxy?

Connection Refused is the server actively refusing your connection, and as you've tried with several different IRC clients I doubt it's an mIRC program. As you're fairly sure it's not your ISP I guess there's something wrong in your computer somewhere along the line! You might try upgrading to the latest version of mIRC (6.15).

Just an FYI, you only have 2 hours to edit your post from when you first posted it smile