The below snippet will provide you with a systems full user name.

Source Code

' @Author - Alexander Bolte
' @ChangeDate - 2014-10-13
' @Description - Returning the system user name.
Public Function getFullUserName() As String
    Dim WSHnet  As Object
    Dim UserFullName As String
    Dim userName As String
    Dim UserDomain As String
    Dim objUser As Object
    Set WSHnet = CreateObject("WScript.Network")
    userName = WSHnet.userName
    UserDomain = WSHnet.UserDomain
    Set objUser = GetObject("WinNT://" & UserDomain & "/" & userName & ",user")
    UserFullName = objUser.FullName
    Set WSHnet = Nothing
    Set objUser = Nothing
    getFullUserName = UserFullName
End Function

Referenced APIs

Since the source is using late binding it is not necessary to set a reference in the VBA Editor for a specific version of the referenced API named "WScript".