One more interesting read on Microsoft XP SP3 installation by Mark Dormer, MVP.

Windows XP SP3 breaks Windows Update

If you install XP SP3 on a clean SP2 box Windows Update will no longer work. Unless you have the newer Windows Update Agent 3.0 your stuck in a hole. There is no way to get it via AU or WU or MU only a manual installation can resolve the issue

1. Download the new Agent (but do not Run it yet) to your Desktop (so you can find it below) :

X86 Version Download.

2. Start, Run, CMD

In the black box:

net stop wuauserv

“%userprofile%\Desktop\WindowsUpdateAgent30-x86.exe” /wuforce (don’t forget the quotation marks)

net start wuauserv exit

The original post.

cheers

Romanus