Problem Launching a portable app.

I'm having a problem getting my application to load. This is most likely my lack of understanding of how the "chrome://" works. I have set up a simple XUL file to act as my application. It is the find file example from the XUL tutorial. Below is the directory structure I'm using:

F:\PortableApps\XULApps
\XULRunner
\CARS (My App)
Start.BAT (used for starting the application)
Application.ini
\Chrome
chrome.manifest (which is empty)
\CARS
Appstart.xul (the application)
\defaults
\preferences
prefs.js (which has the following line (pref("toolkit.defaultChromeURI", "chrome://chrome/cars/AppStart.xul");)

IF I put the full path and use File:// instead of chrome:// everything works correctly. Can you tell me what I'm doing wrong or direct me to a place that may have the info.

Thanks