How do I resolve this problem ,Application showing an exception“file not found ”

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WinForms

C#3.5

I created an application in VS2010 with operating system WINDOWS XP.
Now, I updated the os to WIN 7 and also updated the location of the application.
So, while running the application for opening a file using open dialog box it showing some exception like"File Not Found".
It was working fine with WIN XP, but now it showing this error, if we keep that perticular file in bin folder its working fine , but if we choose a file from other drive or a folder it showing error.

<pre lang="c#">
 string chosen_file = "";
    ofd.Title = "Open a file";
    ofd.FileName = "";
    ofd.Filter = "Text Files(*.txt)|*.txt|Rich Text Box(*.rtb)|*.rtb|Word Document(*.doc)|*.doc|HTML Pages(*.htm)|*.html|Cascading Style Sheet(*.css)|*.css|JAVA(*.java)|*.java|video file(*.wmv)|*.wmv|All Files(*.*)|*.*";
    if (ofd.ShowDialog() == DialogResult.OK)
    {
        chosen_file = ofd.FileName;
        // richTextBox2.LoadFile(chosen_file, RichTextBoxStreamType.PlainText);

        var fileInfo = new FileInfo(ofd.FileName);
        fileInfo.Length.ToString();
        byte[] buffer = new byte[fileInfo.Length];

        int length = (int)fileInfo.Length;
        FileStream fileStream = new FileStream(fileInfo.Name, FileMode.Open, FileAccess.Read);
        fileStream.Read(buffer, 0, length);}