Maybe something like:

'VOG II,http://www.mrexcel.com/forum/showthread.php?p=1666961
Private Sub Worksheet_Change(ByVal Target As Range)
Dim NR As Long
If Intersect(Target, Range("F15:F25")) Is Nothing Then Exit Sub
With Sheets("Log")
.Unprotect Password:="xyz"
NR = .Range("A" & Rows.Count).End(xlUp).Row + 1
.Range("A" & NR).Value = Target.Address(False, False)
.Range("B" & NR).Value = Now
.Range("C" & NR).Value = Environ("username")
.Range("D" & NR).Value = Target.Value
.Range("D" & NR).NumberFormat = Target.NumberFormat
.Protect Password:="xyz"
End With
End Sub