XP computers and Citrix

We have recently encountered problems with Windows XP Pro computers that access our network through Citrix. Suddenly, when the user logs on to Citrix, everything runs very slow, when there are been no problems previously. There will be a very noticeable delay in opening/closing files, and for typed characters to actually appear on the screen. I searched Microsoft web site and found an article regarding a hot fix for this very issue. But the hot fix made the problem worse and we had to reformat the computer’s hard drive. After that, everything works fine. Has anybody else experienced this and if so, how did you solve it?