Const adTypeBinary = 1
Const adTypeText = 2

' accept a string and convert it to Bytes array in the selected Charset
Function StringToBytes(Str,Charset)
  Dim Stream : Set Stream = CreateObject("ADODB.Stream")
  Stream.Type = adTypeText
  Stream.Charset = Charset
  Stream.Open
  Stream.WriteText Str
  Stream.Flush
  Stream.Position = 0
  ' rewind stream and read Bytes
  Stream.Type = adTypeBinary
  StringToBytes= Stream.Read
  Stream.Close
  Set Stream = Nothing
End Function

' accept Bytes array and convert it to a string using the selected charset
Function BytesToString(Bytes, Charset)
  Dim Stream : Set Stream = CreateObject("ADODB.Stream")
  Stream.Charset = Charset
  Stream.Type = adTypeBinary
  Stream.Open
  Stream.Write Bytes
  Stream.Flush
  Stream.Position = 0
  ' rewind stream and read text
  Stream.Type = adTypeText
  BytesToString= Stream.ReadText
  Stream.Close
  Set Stream = Nothing
End Function

' This will alter charset of a string from 1-byte charset(as windows-1252)
' to another 1-byte charset(as windows-1251)
Function AlterCharset(Str, FromCharset, ToCharset)
  Dim Bytes
  Bytes = StringToBytes(Str, FromCharset)
  AlterCharset = BytesToString(Bytes, ToCharset)
End Function

dim s1,s2,FromCharset,ToCharset
s1 = "我的字符串"
FromCharset = "GB2312"
ToCharset = "ISO-8859-1"
s2 = AlterCharset(s1,FromCharset,ToCharset)