Microsoft released a critical update to .Net Framework, and I found that it would not update on my system. After a lot of time and trouble trying to uninstall it and install it I found that the problem was with IIS (Internet Information Services) which is part of XP Pro. For some reason the .Net Framework install needed to stop the service but could not. I was also unable to stop the IIS with the admin services control panel. I was able to get it to update by using task manager to kill the process (end task). I suspect that my Norton Internet Security was protecting the service from ending.
P.S. I use an enhanced task manager that makes this easy to see. Please check out my blog on TaskInfo 2003 for more information.