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.
Windows Update Is Not Installed - WinXP
http://support.microsoft.com/default.aspx?scid=kb;en-us;811269
You are using a version of the AOL software earlier than AOL 8.0 Plus.
-and-
You use the AOL Web browser interface to access and download Windows Updates.
The AOL Web browser that is included with versions of the AOL program that are
earlier than AOL 8.0 Plus did not communicate correctly with the Windows
Installer software. As a result, Windows Installer may stop responding.
Windows Update Troubleshooter
http://v4.windowsupdate.microsoft.com/troubleshoot/
Updates fail to install and you receive error message 0x80070643 - WinXP
If still no satisfaction, post to the WU NG stating your OS and what you've
already tried.
Communities
http://support.microsoft.com/newsgroups/