I have the same thing happen a fair bit at work with firefox - if it crashes there can still be an instance running and you need to kill the process or reboot the pc. I still prefer firefox to explorer at work though.
Have you ever watched a tv show called "The I.T. Crowd" Al? Good old Brittish humour, and with a valuable lesson for anyone diagnosing a computer related problem:
Have you switched it off and on again? 