touch&go,
Here is what Microsoft has to say about it:
Errors Occur When You Try to Install Updates from the Windows Update Web Site
SYMPTOMS
When you use the Windows Update Web site to install updates, you may receive a 643 error message or a message indicating that Windows Update was unable to complete the operation.
Microsoft Windows Media Player may also be slow to respond.
Additionally, the %WINDIR%\system32\CatRoot2\Edb.log may grow to up to 20 megabytes (MB) even though the file is typically only 5,120 kilobytes (KB).
CAUSE
This issue may occur if the catalog database (Edb.log) is damaged. The catalog database is referenced by Windows Update and Windows Media Player. If the catalog database is damaged, the update may not install correctly, and you may experience the behavior described in the "Symptoms" section of this article.
WORKAROUND
To work around this issue, follow these steps:
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 643 Error and the Catalog Database
SYMPTOMS
You may experience the problems that are described in the following Microsoft Knowledge Base articles:
Errors Occur When You Try to Install Updates from the Windows Update Web Site
RESOLUTION
To resolve these problems, obtain the Q817287: Critical Update from the Microsoft Windows Update Web site:
http://windowsupdate.microsoft.com
The English version of this fix has the file attributes (or later) that are listed in the following table. The dates and times for these files are listed in coordinated universal time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.
Date Time Version Size File name
---------------------------------------------------------------------------------
25-Mar-2003 21:40 5.1.2600.1190 53,760 Cryptsvc.dll
25-Mar-2003 21:40 5.1.2600.1190 165,888 Cryptsvc.dll 64-bit
25-Mar-2003 18:54 5.1.2600.1190 53,760 Wcryptsvc.dll x86 for 64-bit package
Note The Q817287: Critical Update cannot be removed.
If this solution does not work, your best bet would be to re-install WinXP Pro w\SP1.
Take Care,
Richard