I don't know what would be causing this. Try making sure you have no other applications open.
It is possible a virus may be causing this (they often cause strange, erratic behaviour) - Look at
this page for ideas.
If it isn't any of the above, try reinstalling mIRC and make sure you get the .exe file from the only truly legitimate source, which is of course
http://www.mirc.com/get.html.
G'luck.
Regards,
