Apparently you might need to edit the registry,
to update to some of the later XP SP2 builds.
MS has issued a registry key with the changes needed called: ssrk_edw.reg

Can someone please provide this file or the content ?

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\WindowsUpdate\M5Deployment]
"XPSP2"="6E7EA86C-BBC2-476A-8C68-FA5474577712"

Your not a beta tester? WTF

But yea sure.

Oh and if you are guessed you just messed it. Go to windowsbeta.microsoft.com and downloads and then xpsp2 and then the Utilities folder there is the ssrk_edw.reg.txt file. :)

Which is this:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\WindowsUpdate\M5Deployment]
"XPSP2"="6E7EA86C-BBC2-476A-8C68-FA5474577712"

PS: Update: The express installer has been taken down from Windows Update V5 and a new one will be posted later this week, according to an announcement made on the beta newsgroups. Because of this, updating to the new build using the initially released express installer is not possible at this time!