You said that a message popped up with "your name as the sender". Do you mean that your regular nickname is sending the message, or that another nickname (your real name possibly) is sending the message? If it is your regular nick sending the message, then your regular mIRC client has malicious code running in it. If it is another nickname sending the message, then you likely have a 'hidden' mIRC (a virus) installed and running on your system. If you close your regular mIRC client, and then hit ctrl+alt+del and view the processes tab, you should see no mirc.exe processes. If there is a mirc.exe process, that is likely the infected program. Terminate it and then use the windows search to locate the directory (search for mirc.exe) and delete the folder that you don't recognize (the one that isn't your regular mirc client).
-genius_at_work