Runtime Crashes - 2.49b

So, sometime it crashes while i move/look around and every time crashes when I press escape. But I hv same file with another model which does not crashes. It start happening during modeling. So, I am confused, what could be the reason. Not only my file buy there are some other files (which I downloaded from net) has also the same problem. It Crashes.
Can anyone tel me some reason that, why runtime crashes.