After installation from .MSI installed application automatically starts up.

How to prevent this for using in unattended setup?

Application is VyPress Messanger 4.0 but I think it's unimportant

this is either a custom action or a installer property, neither of which can be determined without looking at the MSI file itself.

couldnt you just taskkill it?

Like Alanoll said, this is definitely a custom action. If the custom action is in the property table (public), you can modify it thu command line. If not, download tool that creates mst, then download a tool that edits the mst. Then remove the custom action.

lilweirddude's suggestion is another approach. And I think much easier.