You can do this:

vb.net Code:
  1. Dim fs As New FileStream("C:\somwhere\on\your\disk.log", FileMode.Create, FileAccess.Write)
  2.         Dim s As New StreamWriter(fs, System.Text.Encoding.Default)
  3.         s.BaseStream.Seek(0, SeekOrigin.End)
  4.         s.WriteLine("Something")
  5.         s.Close()

Just remember to import the System.IO namespace