@D_T: The error message that panthercules posted is very helpful, especially P9.
A q: do you have your app set as 'Make single instance application' on the Project Designer Application page?
If so, that will be why. The first instance listens through a local network port with subsequent instances passing parameters to the first instance via this port (before exiting again). In this case McAfee is blocking teh access. BTW McAfee and .NET don't play well together.
IIRC there should be an option to stop listening to the port. Try that.
If you don't have make single instance selected, I have some other thoughts, but this is usually the most common.