If Updates fail to install:-
If you receive a 643 error message or a message indicating that Windows
Update was unable to complete the operation. This may occur if the catalog
database (Edb.log) is damaged.
Try this:-
Click Start, click Run, type cmd, and then click OK.
At the command prompt, type the following command, and then press ENTER:
del /q "%SystemRoot%\System32\Catroot2\Edb.log
This command will force the catalog database to rebuild itself.
http://support.microsoft.com/default.aspx?scid=kb;EN-US;817753
Errors Occur When You Try to Install Updates from the Windows Update Web
Site
If the Windows Update scan feature reaches 100%, and then displays "There
are no updates available at this time" message.
Verify that your computer's date and time are accurate and that the Internet
Explorer language option is not empty.
To check the language setting in Internet Explorer:
1. Click "Tools" and then choose "Internet Options"
2. Click the "Languages" button
3. Make sure at least one language is listed in the "Language Options"
dialog.
Windows Update Site Lists Updates That You Already Installed
Windows Update Troubleshooter
http://v4.windowsupdate.microsoft.com/troubleshoot/
Why can't I download updates?
http://www.kellys-korner-xp.com/top10faqs.htm
Windows Update Is Not Installed
http://support.microsoft.com/default.aspx?scid=kb;en-us;811269
This issue may occur if you use the AOL Web browser interface to access and
download Windows Updates. The AOL Web browser does not communicate properly
with the Windows Installer software, and this may cause Windows Installer to
stop responding.
You can download them from the Catalog and save to be installed later.
http://v4.windowsupdate.microsoft.com/catalog/en/default.asp
How to Download Windows Updates and Drivers from the Windows Update Catalog
http://support.microsoft.com/?kbid=323166