Option Explicit

Private Declare Function GetUserName Lib "advapi32.dll" Alias _
     "GetUserNameA" (ByVal lpBuffer As String, nSize As Long) _
      As Long

Function UserName() As String
  Dim strName   As String
  Dim nSize     As Long
  Dim lngResult As Long

  nSize = 100
  strName = Space$(100)

  lngResult = GetUserName(strName, nSize)
  If lngResult <> 0 Then
    UserName = Left$(strName, nSize - 1)
  End If
End Function

MsgBox "Username: " & UserName