Email Button Not Creating new Outlook mail message for some users

I've created a simple form using the LCD 8 / New Form Assistant w/ just an email button. For most users (12) that click the email button, a new Outlook mail message is created. However, for a couple of user clicking the email button results in nothing; no new Outlook mail message, no error.
 
All users have the same base applications installed: WinXP SP2, Adobe Reader 9, MS Office Outlook 2007, etc.
 
What should I look for in trying to determine why some users can't get the new Outlook mail message to pop when clicking on the email button. Is this a security issue w/ Outlook or Reader?