I couldn't get auto-update to work in OfficeXP and MS advised the following registry edit which worked:
"REGISTRY EDITING
1. Click on 'Start' -> 'Run'
2. Type 'regedit' (without the quotation marks) and click 'OK'. The Registry Editor should start.
3. Navigate to the required registry keys by double-clicking on each key.
HKEY_LOCAL_MACHINE\Software\Classes\Installer\Products
4. Below the 'Products' key, select each 32-character {GUID} until you find the one whose 'ProductName' value matches your version of Office. For example, the 32-character {GUID} for Office 2000 Premium is 904000001E872D116BF00006799C897E.
5. With the 32-character {GUID} for your Office product selected, click on 'Edit' -> 'Delete'.
6. Click 'Yes' when prompted to confirm the deletion.
7. Click on 'File' -> 'Exit'.
8. Run Setup for Office again."
Not sure if that will be of any help to you?