Note that the exception indicates that it's an "access violation", meaning mIRC tried to access invalid memory. Again, I've never seen mIRC do this on its own, so I feel like there was some external influence.

On a related note, this might help diagnose: http://www.sevenforums.com/crashes-debugging/210693-appcrash-exception-code-c0000005.html

Heed the warning about overclocking. This goes back to my point about hardware. Did you by any chance overclock your machine? OCing can have this kind of effect on programs. Some modern CPUs have automatic OC detection and will up their clocks automatically by default, check your BIOS to make sure that your CPU is running at its factory specification.