I have recently upgraded about 20 computers from XP Home (various SPs) to XP Pro SP2. After the upgrade many of them started receiving errors while trying to install Windows and/or Office updates from the Microsoft Update site.
I have gone to download then install a single update and it also fails. ( with Microsoft's always helpful, I dunno!?! error). If I download a full version of software and install it, it works. I have also gone to download any software that requires me to check the authenticity of my software prior to download and it passes.