I'm on a mission
because I have so many times seen advice to install reg-cleaners, fix-it tools and scan for malware.
A malware scan is a good idea to start with.
The general fix for this problem is to download the standalone "Internet Explorer 8" Setup from Microsoft manually using a modern day OS(with up2date certificates and crypto) and Computer and put it onto the hopefully virtualized XP, install it and most likely you can again run windows update.
Basically it's a certificate error, the frame where the error occures is https while the Windows-Update start page isnt.
If the error perists it is also good to downgrade the IExplorer security settings for activeX modules because the WindowsUpdate is ActiveX. When you are done reset the degradation and keep the wandering security risk off the internet forever.