Media Center program missing after Vista Express Upgrade

Hey,

I upgraded from XP Media Center edition to Vista using the Express Upgrade from Dell.  The installation went fine, and Media Center program was working beautifully.  The next morning, after booting my computer, the Media Center program would not open and this message popped up: 

"Windows cannot open this program because it has been prevented by a software restriction policy.  For more information contact your system administrator." 

I can't find the MC program in any folder anywhere!  This is my home computer, and I am the only one with admin privleges.  So far, Dell has NOT been very helpful!  Has this happened to anyone else?

Thanks!!!