It does seem ridiculous that Windows XP has been around for quite so long. Of course, you should be thinking about moving away from Windows XP as soon as possible since the extended support period is ending in April 2014, but there are still a huge number of XP machines around. (I'm really keen to migrate the remaining machines in my care sometime this month.)
In the meantime, there's a problem with Windows Update on XP machines
which means that just checking for updates takes an incredibly long
time to run (if it actually completes, I've never waited that long to
see…) and causes svchost.exe to peg the CPU core it's running on at
100% usage.
This is actually reminiscent of a problem from a few years back, although I'm not particularly nostalgic about it. As a result, it's a little more difficult to find relevant search results for, unless you restrict your searches to within the past month or year.
From the sounds of it, there's something rather horrible going on behind the scenes with how the Windows Update client actually determines which patches are required.
Microsoft have been looking into it, but not actually resolved it yet. In the meantime, a fix is to manually install the latest Cumulative Security Update for whichever version of Internet Explorer is installed. Providing this is installed before the check for updates happens, it removes the need for the checks that are hogging the CPU and everything works well again. Someone's been helpfully posting links to the latest updates in this forum thread, otherwise you can just visit Microsoft's TechNet site directly and find the updates in the latest Security Bulletin.
This will have to be carried out every month until Microsoft actually deploy a working fix. On the other hand, at most, there will only be another four sets of monthly updates to do this for, since April will be the last Patch Tuesday for XP.
Just another reason (as if you needed one) to be upgrading from XP though…