Fresh install of XP SP2 immediately updated to SP3 (Also, Office 2007 SP1 installed).

After, both Windows Update and Microsoft Update detect updates but fail to install any.  No error codes or entries in the event logs.  No info in WU Update History (nothing ever installed by this method).

Fix is install Windows Update Agent 3.0.

Save this to desktop then run the following from a .cmd file:

net stop wuauserv
"%userprofile%\Desktop\WindowsUpdateAgent30-x86.exe" /wuforce
regsvr32 "%windir%\system32\wup2.dll"
net start wuauserv
exit

See

http://www.broadbandreports.com/forum/r20453460-XP-Pro-Updates-Fail-after-Fresh-Install-and-XP-SP3-Loaded