I think it is done before multiple times before but for the heck of it...

;Remove if you dont want to check everytime mirc starts
on *:Start:Checkupdate
alias checkupdate {
  .timertimeout 1 15 scripttimeout
  .sockclose mircupdate
  %mircuptodate = false
  sockopen mircupdate mirc.com 80
}
on *:sockopen:mircupdate:{
  sockwrite -n mircupdate GET /get.html HTTP/1.0 $+ $crlf $+ $crlf
}
on *:sockread:mircupdate:{
  if ($sockerr > 0) return
  var %temp
  sockread %temp
  if ($version isin %temp) %mircuptodate = true
}
on *:sockclose:mircupdate:{
  .timertimeout off  
  if (%mircuptodate = true) echo 3 -a Your mIRC is up to date!
  else .timer -m 1 1 mircnotuptodate
}
alias -l scripttimeout {
  .sockclose mircupdate
  if ($?!="Update timed out!!! Check the site for updates?") run http://www.mirc.com/get.html
}
alias -l mircnotuptodate {
  if ($?!="Your mIRC is [NOT] up to date!!! Update now?") run http://www.mirc.com/get.html
}