Just enter the same registration information into the new copy if it doesn't automatically pick up that it's registered. If you can't find that information, go to http://www.mirc.com/reglost.html
and you can have it sent to you again. If you changed email addresses, then you can also email email@example.com and he can verify that you did pay and resend you your information.
As for your errors... it's going most likely be an issue on your end... your network, your router, your firewall, your modem, your network cables, or even your ISP. Your next step isn't some simple thing someone can tell you. You're going to have to troubleshoot all of those items.