Though Microsoft has withdrawn support for Windows XP, a German blog has posted a tweak that could provide support for the next five years
Here are their instructions
Create a text file with this content::
Save as .reg file and doubleclick to import it.
POSReady2009 Updates will now install on a normal XP and Windows Updates shows also the updates for POSReady2009.
The blogger says: ATTENTION: Use it you own risk! These updates are not tested on a regular XP system and could damage your system.
Get the full instructions from Getting XP updates – Tutorials – Home of the Sebijk.com.