on your machine in mirc type /SERVER irc.servername.net
*** dont be typing servername but put in the name of the server you want ***
If that doesnt work go to your brothers machine start a dos session [start]/[run]/[cmd] and type PING irc.servername.net
Get the IP address xxx.xxx.xxx.xxx of the server and then try that on your pc /SERVER xxx.xxx.xxx.xxx
If this works, it looks like your DNS is either busted or damaged, if its just damaged you might try [start]/[run]/[cmd] and type IPCONFIG /flushdns
If its busted well you got bigger problems than just mirc.