Just press Alt-R and see what scripts are loaded. If you find any that you don't recognize, then unload them. It's unlikely that anything not loaded into mIRC in Alt-R is going to be a mIRC issue. Trojans that use mIRC use scripting in basically all cases, though I'm sure someone managed to control mIRC from outside using DDE or COM or some kind of send keys program or whatever... but that's going to be rare to find (if at all).