Code:
Public Class Form1
    Dim a, b As Integer
    Dim newPoint As New System.Drawing.Point()

    Private Sub ab_MouseDown(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseDown
        a = Me.MousePosition.X - Me.Location.X
        b = Me.MousePosition.Y - Me.Location.Y
    End Sub

    Private Sub ab_MouseMove(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseMove
        If e.Button = MouseButtons.Left Then
            newPoint = Me.MousePosition
            newPoint.X = newPoint.X - a
            newPoint.Y = newPoint.Y - b
            Me.Location = newPoint
        End If
    End Sub
End Class