If you are getting advertisements popping up then it's more likely 'spyware'. Try running anti-spyware programs, a few suggestions are listed here
aswell as tips for staying safe in the future. I recommend you read all
of that post.
Spyware is most commonly gained from installing certain programs on your computer, most notably KaZaa and iMesh, two file sharing programs. They tend to cover them up with a nice phrase such as "free advertisements" - don't download them, don't install them. You could also get it just by opening emails or filling out online forms. Downloading mp3s or movies from warez sites that claim to have the latest warez files (they don't).
If you think it's definitely a *virus* though (there's a difference between viruses and spyware) then run 2-3 virus scanners and a couple of trojan scanners too. Running only one is not always affective in cleaning your computer. Are you sending any advertisements to other users? Are you getting kicked from channels for 'spamming' or advertising? If not, an 'mIRC virus' is less likely.
Make sure that in future you do not click on URLs that promise you "free porn" or type anything that promises "free ops". Also, don't accept any files from people that you do not know.