С помощью функций OemToChar и CharToOem:

Declare Function OemToChar Lib "user32" _
    Alias "OemToCharA" _
    (ByVal lpszSrc As String, _
    ByVal lpszDst As String) As Long

Declare Function CharToOem Lib "user32" _
    Alias "CharToOemA" _
    (ByVal lpszSrc As String, _
    ByVal lpszDst As String) As Long

Function WinToDos(ByVal Src As String) As String
    Dim sRet As String
    sRet = Src
    CharToOem Src, sRet
    WinToDos = sRet
End Function

Function DosToWin(ByVal Src As String) As String
    Dim sRet As String
    sRet = Src
    OemToChar Src, sRet
    DosToWin = sRet
End Function