***PLEASE READ***:
This fix is for people that are familiar with registry editing using Regedit only. If you do not have experience using Regedit, I recommend that you do some reading first as you could cause some serious problems on your computer if you modify the wrong keys or values.
1-Close all running Microsoft Office components.
2-Open Regedit
3-Go to: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Common\General
4-Modify the "ShownOptIn" value from 1 to 0
5-Launch a Microsoft Office component, it will give you the option to download the updates, make sure you select it.
6-Done. If you go back to "Windows Update", then "Change Settings", you will see the "Microsoft Update" and "Software notifications" sections are available again (until the Microsoft Update check box is unchecked again).