I did not see this posted anywhere here so thought it worth posting.
XP Service Pack 3 blocks .NET security patches (http://windowssecrets.com/2008/12/04/03-XP-Service-Pack-3-blocks-.NET-security-patches/?n=story1)
It was in todays Windows Secrets newsletter. (12-04-08)
patsywhitegurl's questions on this were moved to the "Computer Problems, Questions and Solutions!" area: http://www.landzdown.com/index.php?topic=29655.0
If you are encountering this issue, please see the following Knowledge Base article for solution steps. Also, you can contact Customer Service and Support (CSS) for no-charge support by calling 866-PC-SAFETY in the United States and Canada. Others can contact their local Microsoft subsidiary - contact informaiton is available at the International Support Web site: http://support.microsoft.com/common/international.aspx
Microsoft Knowledge Base Article 923100
When you try to install an update for the .NET Framework 1.0, 1.1, 2.0, 3.0, or 3.5, you may receive Windows Update error code "0x643" or Windows Installer error code "1603"
http://support.microsoft.com/kb/923100
Thanks,
Aaron
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