http://www.microsoft-watch.com/article2/0,4248,1037305,00.asp
Here's a link that covers the story about how XP Patch 811493 can slow some XP systems down to a crawl. I know I've been fretting because my 2-week old system suddenly went from a speed demon to a sloth. It took it forever to do anything. After uninstalling the patch, my machine is back to its wonderful speedy self.
I'm not recommending that anybody uninstall the patch, just passing along information so that you can make an informed decision...