Creating service which even runs during windows user is logged off

I have my own vb6 application which i used by lot of customers .I already have an update.exe with my application which checks for updates and installs it if available.The checking for updates is done during opening and closing of software. This update.exe is also a small vb6 application which we run by calling it from our Main application.

Now we have requirement where if a user is logged off then also we need to check for updates and install it.I think it can be done by creating service which runs continuously. Now how to create a service which runs my update.exe continuously even when user is logged off?