How do I find the unknown program enabled during Start-Up?

You can go to Microsoft Downloads, SysInternals and get Autoruns 64-bit (download it here). Download it, install it and run it and see if it shows you the program on your computer.

For your specific problem, access the "Logon" Tab.

Yellow highlighted sections in the Autoruns display are errors and can be deleted.

That should solve your problem.

You can rightclick on an entry and click Open file location wich leads you to the directory where the program is installed.

As mentioned in the comments by Ismael Miguel this answer only applies to Windows 8, Windows 8.1 and Windows 10 (and the Windows Server variants). For Windows 7 and older, you need to use msconfig.

Start-up entries are in the registry at Software\Microsoft\Windows\CurrentVersion\Run.

There is one of these for each user and machine. For the current session that would be under HKEY_LOCAL_MACHINE and HKEY_CURRENT_USER.

Any shortcuts in a Start Menu group called Startup (Microsoft\Windows\Start Menu\Programs\Startup) will also be run. Again there are both per-user and per-machine locations, under %AllUsersProfile% and %AppData% for the current session.

Additionally, a scheduled task can be set to run at boot or login. This data is also in the registry Microsoft\Windows NT\CurrentVersion\Schedule, but in a much more complex format. This can be explored with the "Task Scheduler" app.