To receive these updates for your WindowsXP computer, you need to alter a registry entry to fool the Microsoft Update server into thinking that you are using Windows Embedded POSReady 2009.
Here is a .reg file that looks like this if you view it in a text editor:
Windows Registry Editor Version 5.00
Download and double-click on the .reg file, then just run Windows Update as usual.
Not all the available updates are useful for you; there's no need to select the updates for Windows Server 2003, for example. However, even if you do they will just fail to install. This means that Automatic Update will continue to work as before until 2019 (You do have that turned on, right?) and Microsoft Security Essentials will continue to update (you do have that installed, right?). You should be ready for a new computer by 2019 anyway.
If you have a 64-bit Windows XP installation, look here for the appropriate instructions.
One last note. It's not likely that any of the future updates of Windows Embedded POSReady 2009 will update the Internet Explorer web browser and newer version of the browser are not compatible with WindowsXP. Sadly for all of us, Internet Explorer is the target (or the vector) of most all of the malware attacks. Here's how to protect yourself: DO NOT UNDER ANY CIRCUMSTANCES CONTINUE TO USE INTERNET EXPLORER. Replace it with the most recent edition of the Firefox browser and install the AdBlock extension. Most malware nowadays is installed through Internet Explorer via malicious ads that you are completely unaware of.
There are also attack vectors through Microsoft Office. A free and capable alternative is LibreOffice.
An excellent free and capable email application and alternative to Outlook Express is Thunderbird.
I also highly recommend three additional programs for Windows XP:
Piriform's CCleaner to clean up unnecessary files on your drive, since Windows loves lot's of free space on the drive and doesn't work well when you fill up more than about 2/3 of the disk. Internet Explore leaves a lot of useless files that never seem to get deleted. Get the Free Download version and DO NOT install any toolbars when prompted.
Piriform's Defraggler to defragment your drive after you clean it up, since Windows loves a defragmented drive. Get the Free Download version and DO NOT install any toolbars if prompted.
The free Microsoft utility Page Defrag. This authentic Microsoft utility will defrag the special system files that a regular defrag application like Defraggler can't touch. You only need to run this once in a while.
Try them all; you'll be a happier WindowsXP user.
UPDATE: Sadly, Microsoft seems to have fixed this loophole.