I would backup your data, and reinstall FRESH your OS, theres no way mirc should be able to crash the whole pc by just downloading a file, its more likely something unstable in the pc, like a hardware driver thats got corrupted. Mirc is just the program that happens to use/access what ever it is thats broken and cuases the crash.

aka If nails are thrown all over a road, and the same/only car keeps driving down the road and blowing out tires, do you say theres something wrong with the car or the road?