When running Idle on Windows XP, Python 3.1rc2, 
a failure to find an entry in the help documents causes Idle to exit
(with no visible indication of why).  The reason is in a failure of the
call to os.startfile, and apparenly the exception causes Idle to exit
silently.  Here is a patch to .../Lib/idlelib/EditorWindow.py:

@@ -436,20 +436,24 @@ class EditorWindow(object):
     def config_dialog(self, event=None):
         configDialog.ConfigDialog(self.top,'Settings')

     def help_dialog(self, event=None):
        
fn=os.path.join(os.path.abspath(os.path.dirname(__file__)),'help.txt')
         textView.view_file(self.top,'Help',fn)

     def python_docs(self, event=None):
-        if sys.platform[:3] == 'win':
-            os.startfile(self.help_url)
-        else:
-            webbrowser.open(self.help_url)
+        try:
+            if sys.platform[:3] == 'win':
+                os.startfile(self.help_url)
+            else:
+                webbrowser.open(self.help_url)
+        except EnvironmentError as why:
+            tkMessageBox.showerror(title='Document Start Failure',
+                                   message=str(why), parent=self.text)
         return "break"

     def cut(self,event):
         self.text.event_generate("<<Cut>>")
         return "break"

     def copy(self,event):
         if not self.text.tag_ranges("sel"):
@@ -741,20 +745,25 @@ class EditorWindow(object):
         # and update the menu dictionary
         self.menudict['help'] = helpmenu

     def __extra_help_callback(self, helpfile):
         "Create a callback with the helpfile value frozen at definition
time"
         def display_extra_help(helpfile=helpfile):
             if not helpfile.startswith(('www', 'http')):
                 url = os.path.normpath(helpfile)
-            if sys.platform[:3] == 'win':
-                os.startfile(helpfile)
-            else:
-                webbrowser.open(helpfile)
+            try:
+                if sys.platform[:3] == 'win':
+                    os.startfile(helpfile)
+                else:
+                    webbrowser.open(helpfile)
+            except EnvironmentError as why:
+                tkMessageBox.showerror(title='Document Start Failure',
+                                   message=str(why), parent=self.text)
+            return "break"
         return display_extra_help

     def update_recent_files_list(self, new_file=None):
         "Load and update the recent files list and menus"
         rf_list = []
         if os.path.exists(self.recent_files_path):
             rf_list_file = open(self.recent_files_path,'r')
             try: