It is not possible to remotely make a window popup in mIRC via another mIRC client, my guess is you are infected with something. Download, install, update and run several virus/trojan scanners from
this thread. If you haven't already, do not go to the website it asks you to remember. If you have, more of a reason to do the former suggestion.
If nothing is found, it may be an mIRC Script that, to an antivirus, seems harmless. See if the messages stop if you type
/!remote off. Also, press
ALT+R and use the 'Find Text' button to look for possible code that could be causing the error (i.e. search for the text inside the dialog that pops up).
If all else fails, try reinstalling mIRC completely. But do make sure you use more than 1-2 virus scanners, no ONE can detect everything. You may also consider running the #Nohack script (link in that thread) to see if you have some of the 'mIRC viruses' that get spread around. You should always be using the latest version of mIRC, always available at
www.mirc.com/get.html G'luck!
Regards,