"Unable to resolve server" means that mIRC (and therefore Windows) cannot convert the server hostname given (eg. irc.someserver.com) into an IP address to communicate with. This generally means one of three things:
1. You're not connected to the internet. Presumably we can discount that one.
2. The server hostname you've given has a typo in it. Double check you've given the correct server name.
3. The server no longer exists or uses a different hostname. Try visiting the IRC network's website to find a different server for that IRC network.
