Yeah, but that's not a big deal. The users could uninstall the patch and their computers would be up and running at normal speed again.
The big problem is that there are lots of people who are offering fake patches saying that the patches they are offering won't check the registration key, and the patches MS offers will. People who are running stolen versions of XP then download these fake patches only to be infected with a trojan. I've seen it all over usenet and IRC, people need to be much more cautious.
Anyways, just be careful people, I've seen it happen so many times. If you don't download the patches from MS, make sure you scan them before you run them!