Owner of files written by a service

Question

I have a problem with "Windows Server R2" in particular.
UAC is switched on and should remain in this standard state.

I want to work with a domain user "myuser", with admin rights granted.
When I work with "myuser" and create a file with windows explorer its owner is set to "myuser", as expected.

But when I start a service, which was also started with User "myuser", files are always written with owner "Administrators"

Has anyone an idea, how to change this behavior of a service? (User "myuser" has to be admin)
How would it be possible to force the service to write files with user "myuser"?

By the way: UAC-Settings didn't have any impact on the owner of files, written from the service.