RichTextBox...nt getting where is this..plz help

Question

I'm trying to make my ASPX page able to process WORD format document. I
learned that RichTextBox might help me, but I can't use it either by adding
it to my Web Control list or by writing "<asp:RichTextBox id="c1" />" in the
ASPX file. The error message shows "Cannot find RichTextBox in the
namespace of web controls" (something like that). Is there any way to add
RichTextBox in ASPX file? Or where can I find a substitute?

Thanks in advance!