Visual basic

Para conocer que letra tiene la unidad de CD-ROM no queda, al menos que yo sepa, otro remedio que acceder a las API's de Windows para obtenerlo. La función para obtener lo que deseas podría ser la siguiente:
Private Declare Function GetDriveType Lib "kernel32" Alias
"GetDriveTypeA" (ByVal nDrive As String) As Long
Private Declare Function GetLogicalDriveStrings Lib "kernel32" Alias
"GetLogicalDriveStringsA" (ByVal nBufferLength As Long, ByVal
lpBuffer As String) As Long
Private Const DRIVE_REMOVABLE = 2
Private Const DRIVE_FIXED = 3
Private Const DRIVE_REMOTE = 4
Private Const DRIVE_CDROM = 5
Private Const DRIVE_RAMDISK = 6
Function StripNulls(startStrg$) As String
Dim c%, item$
c% = 1
Do
If Mid$(startStrg$, c%, 1) = Chr$(0) Then
item$ = Mid$(startStrg$, 1, c% - 1)
startStrg$ = Mid$(startStrg$, c% + 1, Len(startStrg$))
StripNulls$ = item$
Exit Function
End If
c% = c% + 1
Loop
End Function
Private Sub Form_Load()
Dim r&, allDrives$, JustOneDrive$, pos%, DriveType&
Dim CDfound As Integer
allDrives$ = Space$(64)
r& = GetLogicalDriveStrings(Len(allDrives$), allDrives$)
allDrives$ = Left$(allDrives$, r&)
Do
pos% = InStr(allDrives$, Chr$(0))
If pos% Then
JustOneDrive$ = Left$(allDrives$, pos%)
allDrives$ = Mid$(allDrives$, pos% + 1, Len(allDrives$))
DriveType& = GetDriveType(JustOneDrive$)
If DriveType& = DRIVE_CDROM Then
CDfound% = True
Exit Do
End If
End If
Loop Until allDrives$ = "" Or DriveType& = DRIVE_CDROM
If CDfound% Then
label1.Caption = "El CD-ROM corresponde a la unidad: " & Ucase$(JustOneDrive$)
Else
label1.Caption = "Su sistema no posee CD-ROM o unidad no encontrada."
End If
End Sub