There were a few registry entries &/or changes widely published when Microsoft support ended for XP -- XP Home & Pro were then identified as POSReady 2009, basically what was used in kiosks, ATMs etc. And that POSReady 2009 version of XP has continued to receive updates without a glitch. That is until recently -- the last several months have seen Windows Update taking hours at 100% CPU to ID needed updates.
The cure, which has worked the last few months now, is to download the updates from Microsoft's Update Catalog. If the Windows Update Service is running at 100%, it's necessary to stop that service before you can do much of anything. To install the updates you just run them -- you should be able to check Windows Update afterward to confirm, & that should work normally.
If for whatever reason someone did not want to do the registry mod for XP to report POSReady 2009, it's possible that they may find a way to use these updates as well.
TO find & download the updates, visit the following URLs, probably making sure to sort by the most recent date.
catalog[.]update[.]microsoft[.]com/Search.aspx?q=posready
catalog[.]update[.]microsoft[.]com/Search.aspx?q=internet+explorer+8
Note that the 2nd URL may or may not have anything different to offer -- I've seen it both ways.