First, ensure you have the latest help centre and Photoshop Help package:
Help Centre 2.1: http://www.adobe.com/support/downloads/detail.jsp?ftpID=3171
Help Package: http://www.adobe.com/support/downloads/detail.jsp?ftpID=3088
Ensure that the packages file, Photoshop_9.0_en-us.zip, is in the /Adobe Help Center/AdobeHelpData/Packages folder.
There are a couple of Techdocs relating to the Help Centre:
331717 Help window doesn't appear when you start Adobe Help Center
(Windows XP, 2000)
319210 Adobe Help Center is missing content or returns the error, "The topic you want to view applies to a CS2 product you have not installed."
If those don't help:
1. Try deleting the Help Preference file by holding down CTRL+ALT+SHIFT whilst starting Help.
2. Assuming ahc.exe is located in C:\Program Files\Adobe\Adobe Help Center,
delete the following folders
C:\Program Files\Adobe\Adobe Help Center\AdobeHelpData\Cache
C:\Program Files\Adobe\Adobe Help Center\AdobeHelpData\Database
C:\Program Files\Adobe\Adobe Help Center\AdobeHelpData\Preferences
C:\Program Files\Adobe\Adobe Help Center\AdobeHelpData\Search
The only folder under C:\Program Files\Adobe\Adobe Help Center\AdobeHelpData
you should have left is one called packages.
What should happen is that your help center database should get rebuilt (you
will see progress messages as it process each help package) and everything