Click on start->right click on my computer, select manage. Expand event viewer and look for errors in the system and application log. Double click the error entry to get the details of the error.
In any case, to replace explorer, find another computer that has a working explorer.exe for Vista. Copy it onto a flash drive or cd. Boot into safe mode or console mode and rename your explorer.exe and copy the one from the other computer and see if there is any difference.