example usage:
<% 
Dim strErrEvent

On Error Resume Next
    ... some code Or event that may fail due To an Error
If Err Then
     ' in the event of an error, write the page path, 
     ' error number and error description to the log file...
    strErrEvent =   Request.ServerVariables("PATH_INFO") & vbTab & _
                Err.number & vbTab & Err.description
    WriteLog Server.MapPath("/website_errors.txt"), strErrEvent
End If
On Error GoTo 0
%>
source code:
<%
Private Sub WriteLog(ByVal pathname, ByVal logevent)
    Dim objFSO, objFile
    Set objFSO = Server.CreateObject("Scripting.FileSystemObject")
    Set objFile = objFSO.OpenTextFile(pathname, 8, True)
    objFile.WriteLine Now() & vbTab & logevent
    objFile.Close
    Set objFile = Nothing
    Set objFSO = Nothing
End Sub
%>