How to keep track of cursor positiion in multi-line textbox?

[Menno Abbink]

I'm writing a simple asp.net (with vb.net) guestbook application. I wan't
the users to be able to insert a emoticon in there message by clicking on
the emoticon of there choice. After clicking on the emoticon a little piece
of text will be added to there message (e.g. :smile:) at the exact location
where the cursor was before they clicked the emoticon.

How can I keep track of the cursos position to do this?

TIA.