Private Declare Function apiGetUserName Lib "advapi32.dll" Alias _
                 "GetUserNameA" (ByVal lpBuffer As String, _
                 nSize As Long) As Long
 
Private Function GetUserName() As String
    Dim sBuff As String * 25
    Dim lBuffLen As Long
 
    lBuffLen = 25
    apiGetUserName sBuff, lBuffLen
    GetUserName = Left(sBuff, lBuffLen - 1)
 
End Function

Private Sub Worksheet_Change(ByVal Target As Range)
    
    Dim lRow As Long
    
    On Error GoTo ErrorHandler
    
    'did the user change something in column C?
    If Not Intersect(Range("C:C"), Target) Is Nothing Then
        
        lRow = Target.Cells(1).Row
        
        Application.EnableEvents = False
        Cells(lRow, 1).Value = GetUserName
        Cells(lRow, 2).Value = VBA.Now
        
    End If

ErrorExit:
    Application.EnableEvents = True
    Exit Sub
    
ErrorHandler:
    
    Resume ErrorExit
End Sub