corrupt image when run as shutdown script

I am using version 5.3 64 bit on windows 7.
I have a generated Dos shutdown script with a link on the desktop and this works well. As I want to automate the backup I have edited the local group policy on win 7 to run the script at shutdown.
The problem is that the backup file generated using the group policy approach is corrupt, it is about half the size it should be.

It is not an intermittent problem, if I use the desktop Icon to run the script it always works, if I run the script using the group policy approach it is always corrupt.

Any ideas??