I have a similar problem where I can assign a task in Outlook 2003, the recipient receives it normally, and accepts it. When their reply hits my inbox with the acceptance message, I can preview their reply, but cannot open it. The mouse pointer will momentarily change into an hourglass as if something's happening, but it will not open. I do not get an error message. If I enable the preview pane, I can see the message contents. I can even forward their message, then open its attachment, and voila, a normal "task accepted" form will open up.
I tried looking for the update number you referenced, but that parcular number doesn't appear in my programs list under Add/Remove.
In the auto-preview pane, it acknowledges the message contains "active content". I went into security settings and enabled all active content.
Any other suggestions?