mIRC has never caused a display driver to stop working, and if it did, it still could not be mIRC's fault, but rather the display driver's. You need to update the display driver if it is not the latest driver and verify that you're running no scripts or dlls that could be messing with it.
You say you're an "mirc addict" so I imagine mIRC is always running on your machine.. the fact that "this only happens when mIRC is on" if mIRC is always on doesn't really narrow this down to an mIRC issue.. that's just coincidence.
