Replacing Words In Text File?

I am using the following code, but cannot get this to work. I have got it to work if the replacement is just of one word. So if the word test is replaced by mouse and the only thing in the file is the word test, it works.

How can I replace like this:
CH_IPVOD_filename.mpg > replace the CH_IPVOD_ with nothing for the result of:
filename.mpg
?

I would like to replace every instance of ch_ipvod_ with nothing.
Public Class Form4
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim fso, inputFile, outputFile
Dim str As String
[Code] .....