Two articles about this:

Does your AMD-based computer boot after installing XP SP3?
http://msinfluentials.com/blogs/jesper/archive/2008/05/08/does-your-amd-based-co
mputer-boot-after-installing-xp-sp3.aspx

You receive a "Stop 0x0000007E" error message after you upgrade to Windows XP Service Pack
2 or Service Pack 3 on a non-Intel-processor-based computer
http://support.microsoft.com/kb/888372

Where to get asssitance if the above doesn't help -

Microsoft Windows XP Service Pack 3 (All Languages)
http://support.microsoft.com/oas/default.aspx?ln=en-us&prid=11273&gprid=522131

Free unlimited installation and compatibility support is available for Windows XP, but
only for Service Pack 3 (SP3). This support for SP3 is valid until April 14, 2009. For
more information about this policy, visit the Windows XP Support Lifecycle page located at
http://support.microsoft.com/lifecycle/?p1=3223. Chat and e-mail support is available only
in the United States and Canada. For all other Windows XP issues, visit the Help and
Support site at http://support.microsoft.com/oas/default.aspx?gprid=1173, and then choose
your product.

. US:
http://support.microsoft.com/oas/default.aspx?ln=en-us&prid=11273&gprid=522131

. CA:
http://support.microsoft.com/oas/default.aspx?ln=en-ca&prid=11273&gprid=522131

. UK:
http://support.microsoft.com/oas/default.aspx?ln=en-uk&prid=11273&gprid=522131

. AU:
http://support.microsoft.com/oas/default.aspx?ln=en-au&prid=11273&gprid=522131

. Other:
http://support.microsoft.com/oas/default.aspx?gprid=1173
select Windows XP | select Windows XP Service Pack 3

Finally, there is a NG for Windows Update issues -

Windows Update newsgroup:
(OE newsreader)
news://msnews.microsoft.com/microsoft.public.windowsupdate
or
(Web)

http://www.microsoft.com/windowsxp/expertzone/newsgroups/reader.mspx?dg=microsof
t.public.windowsupdate&lang=en&cr=US