If that box is popping up then that must mean that mIRC is starting up when you start Windows, and then hiding itself, but failing to hide the About dialogue. The most likely
explanation for this (but certainly not the only one) is that you're infected with some sort of virus, and most probably trying to load a GTbot (See this website
is generally considered one of the best programs for trojan and GTbot removal. Otherwise, you can search Google
for "free online virus scans".
Whatever you do, unless you're experienced with computers, do not go killing off processes that you don't understand, get an anti-virus to do it for you if it's necessary. That way, the likelihood of damage being done to your computer is minimal.