Thanks to Evi Hairani

Private Declare Function GetDiskFreeSpace Lib "kernel32" _
    Alias "GetDiskFreeSpaceA" _
  (ByVal lpRootPathName As String, _
   lpSectorsPerCluster As Long, _
   lpBytesPerSector As Long, _
   lpNumberOfFreeClusters As Long, _
   lpTotalNumberOfClusters As Long) As Long

' Get the free space.
Private Sub GetFreeSpace(ByVal drive_letter As String, _
    ByRef total_size As Double, ByRef total_free As Double)

    If GetDiskFreeSpace(drive_letter & ":\", _
        sectors_per_cluster, _
        bytes_per_sector, _
        num_free_clusters, _
        total_clusters) _
    Then
        total_size = total_clusters * sectors_per_cluster * _
            bytes_per_sector
        total_free = num_free_clusters * _
            sectors_per_cluster * bytes_per_sector
    Else
        total_size = 0
        total_free = 0
    End If
End Sub

Private Sub Form_Load()
  cbdrive.ListIndex = 0
End Sub

Private Sub Timer1_Timer()
Dim TotSize As Double
Dim TotFree As Double

' do something
  GetFreeSpace cbdrive.Text, TotSize, TotFree
  If (TotFree / 1024) / 1024 < CLng(Txtsize) Then
    frmTip.Show
  End If
End Sub