Like others I found that Mobipocket Creator wouldn't work properly when I upgraded to Windows 7 and IE9. This is due to the fact that Mobipocket Creator uses html pages as part of its internal processing and these are incompatible with IE9.
However Microsoft have provided a way for people with old html to continue using it in IE9. See the following
The essence of this is that that if, for example, you have old html that works in IE8 and you want it to continue to work in IE9, then all you need to do is insert the following two lines immediately after the <head> statement in ALL the html files in the "C:/Program Files/mobipocket.com/mobipocket creator" folder (i.e. those in the rsc/html folder and the /rsc/html/editor/popups folder)
<!-- Mimic Internet Explorer 8 -->
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8" >
If you want compatibility with a different version of Internet Explorer then modify the lines accordingly, based on the info in the Microsoft web page.
ONLY ATTEMPT THIS IF YOU KNOW WHAT YOU ARE DOING WITH HTML AND MAKING CHANGES TO THE PROGRAM FILES FOLDER!
TAKE A BACKUP COPY OF YOUR EXISTING FOLDERS SO THAT YOU CAN USE THEM TO RESTORE THE ORIGINAL POSITION IF NECESSARY.
NOTE 1: these changes should be done using a text editor such as Notepad and NOT a word processor.
NOTE 2: You do NOT need to add these lines to any html for books you are creating. The change is purely to the html that mobipocket uses to run.
NOTE RE TABLE OF CONTENTS (added 26th April 2012)
The above changes fix the Guide option, but not the Table of Contents option. However I have further investigated the table of contents problem and believe I have now found a solution. To fix this, some additional changes are required. These are explained on the following thread.