Suppose you want your program to behave differently depending on whether it is launched from the Start menu, or by clicking the pinned icon on the taskbar, or by Scheduled Task, or from a service, or whatever. How can a program detect and distinguish these scenarios?
The answer is you don't. And you shouldn't try.
Instead of trying to guess how your program was executed, you should have the launcher tell you how they are executing your program. You do this by registering a different command line for each of the scenarios, and then checking for that command line in the program. (We saw a variation of this a little while ago.)
For example, you could have your Start menu shortcut contain one command line parameter, give the taskbar pinned shortcut a different command line parameter, register yet another command line parameter with the task scheduler, and have the service launch the program with a still different command line parameter.
They all run the same program, but the command line parameter lets the program know what context it is being run in and alter its behavior accordingly.
It's like creating multiple email addresses that all map to the same inbox. Many email services let you take an email address and insert a plus sign followed by anything else you like before the at-sign, and it'll all get delivered to the same inbox. The thing after the plus-sign is ignored for delivery purposes, but you can use it to help organize your inbox, so you know that the message sent to firstname.lastname@example.org is related to your fantasy baseball team, whereas email@example.com is something about your frequent flier account.
One thing you shouldn't do is try to guess, however. Programs that magically change their behavior based on details of the environment lead to problems that are very difficult to debug.
Given this discussion, perhaps you can provide guidance to this customer:
How can my DLL detect that it is running inside a service?