If you ever use Microsoft's Internet Explorer avoid using a file:/ URL as your 'Home Page' in the 'Internet' section of the System Preferences app.
I use OmniWeb 95% of the time and had set my 'Home Page' to a file:/ URL. When I tried to use IE some time later it crashed EVERY time I launched it. No error messages or clues. It had been a while since I made the Home Page change so it didn't register as a possible cause.
Mac OS X Hints