Kixforms can be a bit intimidating if you've never used it before, so here is a quick example using the kixforms classic .dll to get you started.

 Code:
$system = CreateObject("Kixtart.System")

$Form = $System.Form()
$Form.Width = 300
$Form.Height = 200
$Form.Text = "Script Complete"
$Form.Icon = 13
$Form.Resizable = 0
$Form.MaximizeBox = 0
$Form.Location = 200, 200
$Form.BackColor = "Blue"

$Label = $Form.Controls.Add("Label")
$Label.Text = "Script completed successfully."+@CRLF+"Click OK to continue."
$Label.TextAlign = 32
$Label.Width = $Form.Width-25
$Label.Height = $Form.Height-90
$Label.Top = 10
$Label.Left = 10
$Label.ForeColor = "Gainsboro"

$OKButton = $Form.Controls.Add("ToolButton")
$OKButton.Text = "OK"
$OKButton.Top = $Label.Bottom+10
$OKButton.Left = $Form.Width/2-$OKButton.Width/2
$OKButton.OnClick = "$$Form.Hide()"
$OKButton.BackColor = "Gainsboro"

$Form.ShowModal()
While $Form.Visible
   $Nul = Execute($Form.DoEvents)
Loop
Exit 0