"Warning: Page has Expired" error ?

Hi All,
I developed an ASP.NET application using VB.NET.
I get a "Warning: Page has Expired"error message when I return back to my page .

Below is the case.

When user clicked on a HyperLink(Web control), MyFile.doc will show at the same browser windows.

Following is the code to open MyFile.doc (workable code)
========================================
Dim hypLink As New HyperLink()
hypLink.NavigateUrl = "file://C:/MyFolder/MyFile.doc"
hypLink.Text = strText
panel1.Controls.Add(hypLink)


But when user click the IE Back button, I get the error message.
Error message are "Warning: Page has Expired". (I have to refresh the page thenit okay)

Anyone have solution on this issue, why this message pop up ?

How to get IE to open the document(MyFiles.doc) in the new windows with the used of HyperLink web control ?

I know one of the way is by changing the File Type setting from folder Option at client pc.

Thanks.

Regards,
Hean