actually it is not the real OS profile name either wihich is a bug kinda:
it's the first name of the used profile (the one choosen at it's creation)

I have changed it after it's creation.

My profile name is now (let's say) "xy".

Before it was just "x".

The profile folder (in C:\Users\) is still called "x", but my real profile name is different from that.

I would prefer the Steam name as the default character name over the OS profile's one though.

If you want to stay with the OS profile name, then pls use the correct Win API:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms724432%28v=vs.85%29.aspx