Problem receiving tasks in Outlook

I use Outlook 2003 to assign and keep track of tasks in a small office network. Everything worked fine for quite a while until after a "Windows Update". Now when I send an assignment, the recipient gets the email but is unable to accept the assignment. A while back we had the same problem that was solved by removing the particular security update. The security update must have been automatically reinstalled. What I need to know is "which security update is the one that needs to be un-installed, and how do I keep it from installing again? Or, is there another way to correct this problem? Please Help - Thanks