Print2Flash itself does not display any windows at automated conversion. But it uses document applications to perform conversion and they may display windows when sending documents to the printer. This is the case for Adobe Reader, MS Office and other applications as well. When instructing such applications to perform printing Print2Flash tells them not to display any user interface but apparently most applications ignore it.
There is a workaround for this: you need to use Print2Flash OLE Automation API and run it under a different user account. This way document applications may show windows but they are not visible as they are displayed on another (hidden) Windows desktop and not on the interactive user desktop. See this link for more information on this: http://print2flash.com/help/SettingupAu ... ccount.php