End a process

[engnouna]

Hi

I have a TextBox in which the user enters the filename of the file that he wants to display and a Button
I am using the process class to open files of any type(.xls,.pdf,.doc,.xml)...
when the user clicks on a button, it displays the filename mentioned in the textbox,and if he presses on the button for a second time and the textbox has changed,it should close the previous file and then open the new file.
how can I do that so only one file is opened?(If the first file is an excel and the second filename is a txt document, then I should close the excel file and then open the text document)
the code I am using is:

Imports System 
Imports System.Collections.Generic 
Imports System.ComponentModel 
Imports System.Data 
Imports System.Drawing 
Imports System.Linq 
Imports System.Text 
Imports System.Windows.Forms 
Imports System.Diagnostics 

Namespace WindowsFormsApplication3 
    Public Partial Class Form1 
        Inherits Form 
        Private p As New Process() 
        
        Public Sub New() 
            InitializeComponent() 
        End Sub 
        
        Private Sub button1_Click(ByVal sender As Object, ByVal e As EventArgs) 
            p.StartInfo = New System.Diagnostics.ProcessStartInfo(textBox1.Text) 
            p.Start() 
        End Sub 
    End Class 
End Namespace