Windows Vista problem

I was using the Portableapps platform on an Acer laptop with Windows XP with no problems. I have since bought a new HP laptop with Windows Vista and I've started having problems. Everything loads ok and I can use all apps. The main app I use is Gnucash. When I come to close down Portable apps prior to removing my external drive I get a message saying that the computer can't write to the Portableapps platform. Anybody got any ideas ?