Although I certainly don't work for Windows, my gut feeling is that adding the "Critical" adjective in front of anything like "Software Update" is generally a bit exaggerated.
If Microsoft were to ask me "What should we call these updates" I would call the "Updates designed to solve problems that we think might prevent some obscure problem in the future, and that probably won't cause your computer to operate differently in any noticeable way, but might fix some problem you would have only heard about if you were a software developer."
Now I'm not saying that I would call them "useless updates designed as a practical joke to annoy you", but I still think that most all software companies go overboard on how frequently you need to look for updates to their software, microsoft notwithstanding. I also tend to think that the fact that Windows seems to want to update on a daily basis has some lawyers and non-computer business people in its ancestry.
I myself generally don't actually get any updates unless I'm loading some software onto my computer and the new software demands that I get all relevant updates in order to function.
Finally, from within the control panel, there are a number of ways to disable the annoying windows updates. Several settings may have to be changed in order to get your XP system to never ask for an update again, and I'd be happy to direct you through it if you need help finding these settings.
Hope this helps,