Scusami forse era meglio se te lo convertivo gia' in VB.NET

Fai un modulo e metti
codice:
Module Module1
    Declare Function SetWindowPos Lib "user32" (ByVal hwnd As IntPtr, ByVal hWndInsertAfter As Integer, ByVal x As Integer, ByVal y As Integer, ByVal cx As Integer, ByVal cy As Integer, ByVal wFlags As Integer) As IntPtr

    Public Const SWP_FRAMECHANGED = &H20
    Public Const HWND_TOP = 0
    Public Const HWND_TOPMOST = -1
    Public Const HWND_NOTOPMOST = -2
    Public Const SWP_DRAWFRAME = SWP_FRAMECHANGED
    Public Const SWP_HIDEWINDOW = &H80
    Public Const SWP_NOACTIVATE = &H10
    Public Const SWP_NOCOPYBITS = &H100
    Public Const SWP_NOMOVE = &H2
    Public Const SWP_NOOWNERZORDER = &H200
    Public Const SWP_NOREDRAW = &H8
    Public Const SWP_NOREPOSITION = SWP_NOOWNERZORDER
    Public Const SWP_NOSIZE = &H1
    Public Const SWP_NOZORDER = &H4
    Public Const SWP_SHOWWINDOW = &H40


    Public Sub FormOnTop(ByVal frm As Form, ByVal VF As Boolean)
        ' Questa procedura permette di impostare sempre in primo piano un form rispetto alle altre finestre
        ' frm ---> form su cui agisce la procedura
        ' vf  ---> se true il form è visualizzato in primo piano

        Select Case VF
            Case True
                SetWindowPos(frm.Handle, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOACTIVATE Or SWP_SHOWWINDOW Or SWP_NOSIZE Or SWP_NOMOVE)
            Case False
                SetWindowPos(frm.Handle, HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOACTIVATE Or SWP_SHOWWINDOW Or SWP_NOSIZE Or SWP_NOMOVE)

        End Select
    End Sub
End Module
Poi nella form con
codice:
FormOnTop(Me, True) 'la metti in primo piano
FormOnTop(Me, True) 'la rimetti normale