changing ASP TEXT AREA content

I want to insert Bold or Italic tags in the text area (along with the
contents already present during replying to an emai)l when the user presses the bold or italic button present at the bottom of
text area in the form . If I use the code below it makes the already existing text to bold or italic and if I modify the text by
inserting some more text , the text does not show the changes
after I hit the submit button.
somebody suggested inline frame with java script but I want to
do ASP.NET so please suggest something ... to correct this.

Thanks!
kms

Private Sub btnItalic_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnItalic.Click
txtContent.InnerHtml = txtContent.InnerText.Insert(0, "<I>")
txtContent.InnerHtml = txtContent.InnerText.Insert(txtContent.InnerText.L ength, "</I>")
End Sub