Hi,
Thank you for posting.
I know that you cannot run installer with a startup script in Windows Vista. If I have misunderstood you concern, please feel free to let me know.
Based on my research, I would like to suggest the following:
1. To install software with Group Policy, you can try Group Policy Software Installation.
Please refer to:
How to use Group Policy to remotely install software in Windows Server 2003
http://support.microsoft.com/kb/816102
Group Policy Software Installation
http://technet.microsoft.com/en-us/library/cc738151(WS.10).aspx
2. Just for a test, please disable UAC on the Windows Vista client and check if the script will be applied then.
Hope this helps. Thanks.
Nicholas Li - MSFT