My desktop system is a HP8200 with Avid Xpress Studio complete.  I now must be portable.  My question is can I transfer my system complete with dongle to a laptop or is the dongle tied to the desktop computer? 

 No, you are not limited to your desktop. You can use the dongle on your laptop as well.

To elaborate further -- you can have both installs and simply plug your dongle into the one you want to use at any given time. That is the beauty of the dongle scenario...