cosi funziona meglio
codice:
Public Class Form1
    Dim c As String
    Dim start As Boolean
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Button1.Text = "start"
        start = False
        Timer1.Stop()
        ' ogni minuto fa lo screenshoot
        Timer1.Interval = 10000
    End Sub
    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        Dim bounds As Rectangle
        Dim Screenshot As System.Drawing.Bitmap
        Dim graph As Graphics
        Dim percorso As String
        Static b As Integer

        bounds = Screen.PrimaryScreen.Bounds
        Screenshot = New System.Drawing.Bitmap(bounds.Width, bounds.Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb)
        graph = Graphics.FromImage(Screenshot)
        graph.CopyFromScreen(bounds.X, bounds.Y, 0, 0, bounds.Size, CopyPixelOperation.SourceCopy)
        b = b + 1
        percorso = Application.StartupPath & "\" & b & ".jpg"
        Screenshot.Save(percorso, System.Drawing.Imaging.ImageFormat.Jpeg)
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If start = False Then
            Button1.Text = "stop"
            Timer1.Start()
            start = True
        Else
            Button1.Text = "start"
            Timer1.Stop()
            start = False
        End If
    End Sub
End Class