Windows – VB6 application no longer opens on Vista computer

I have a VB6 app that formerly worked perfectly on a Vista machine as a scheduled task, but it will no longer open on the same machine. The app generates export files in a specified folder with no direct output on the screen. I get no errors, no missing references, just absolutely nothing.

The machine is running Vista Business 32-bit, UAC is disabled with a single administrator account, and automatic updates are turned off. The app resides in a non-protected folder, and the export files are put in a folder on the desktop. The client swears that the only change they made to that computer since I installed this app was installing Norton Antivirus, which has never caused problems before with our software.

In addition to the normal VB6 references, the app references Microsoft Scripting Runtime (scrrun.dll), and Microsoft DAO 3.6 (dao360.dll). Both of these files are present and registered on the target machine, along with all the other VB6 dependencies. I added MsgBox statements at the beginning of Sub Main() just to see if anything is being executed, and its not. Disabling Norton yielded no results, nor did reinstalling VB6 runtime to rule out any corrupted libraries. Not once did I get any messages, error or otherwise from my app.

I've never had an issue like this before and I'm completely stumped. Is there anything else that could be causing this?

Edit – The app does not run even when I run it manually, so the part about it being a scheduled task is irrelevant to my problem, sorry for including it.

The user has full administrator credentials, no compatibility mode was needed on the initial test which at the time, was done on this very machine I am having the problem on. For grins I tried compatibility mode for XP and 2000, still nothing.