Go to the Control Panel and uninstall Firefox. Then go to the Temp folder where Firefox .exe files were downloaded to. Delete Firefox 4 file and double-click on the latest of the other .exe files that you should still have there. Also, look in your Recycle Bin. One of your old files might still be there.

I try to always download files from the Mozilla page. That way, I always have backup install files.

It will not allow me to uninstall the program. I get the same message "your computer must be restarted"