A few things to try to narrow it down:
Check the Windows Event Log for crash events
Check the Windows Event Log (in the Application section) for crash reports from your application. You can quickly get to the log viewer on Windows XP by clicking Start > Run, typing eventvwr
and pressing Enter. On Windows 7, you can type "event viewer" in the search box that is in the Start menu. You can filter the events to only show error events from your program.
You might find a few error events on one of the computers where this problem has already occured, because it sounds like the error reporting feature is turned off on these computers (in which case "hard crashes" like access violations are logged in the Event Log instead of displaying an error dialog to the end-user, as long as a debugger isn't installed on the computer).
Make sure "Error Reporting" is turned at the OS level
As mentioned in the previous section, it sounds like the error reporting feature is turned off on these computers. In that case, a crash won't display any kind of message to the end-user at all and the application will just vanish suddenly. In Windows XP, you can check this setting (and turn it on) as follows:
- Right-click "My Computer" and select Properties.
- Open the Advanced tab and click the Error Reporting button.
- Select the Enable Error Reporting option.
- Click OK to all the open windows.
Add trace code to your application
You could also add some trace code to your application's start-up code, such as code to display a message box or write a message to the Windows Event Log or to a log file as soon as your application starts (for example, in the Form_Initialize
event of your main form, or in a Sub Main
routine).
This way you will be able to tell whether your application is crashing before or after the VB6 runtime is loaded: if you try to start the application and it disappears/crashes, and your startup message isn't logged, then you know it's crashing before it even has a chance to get to your application's startup code, which could indicate that a dependency of the VB6 runtime or the VB6 runtime itself is not installed properly.
Note that Windows XP and Windows 7 both ship with the VB6 runtime pre-installed, but it's possible for misbehaving installers to overwrite or remove files that are part of the VB6 runtime.