Funny, I had to do the exact same thing and was not able to find something
like what you are asking for.
I did find one or two companies that sold text editing controls but they
were too expensive for what we needed. I our case we were looking for
something that will be a close equivalent to WordPad.
Anyway, we ended up needing to create our own custom control to do that
but you can find something that would cover the *very* basics here:
http://www.planet-source-code.com/vb/scripts/ShowCode.asp?txtCodeId=4522&lngWId=10
Be aware that the sample code of the link above has some bugs and it does
not behave like Word or WordPad, for example if you have a text selected
that has some bold words on it and you click the italic button then
everything will turn italic and you will loose the bold.
Check it out, if you are doing something simple for internal use it may be
a good starting point, if you are doing this for commercial purposes you
have some work to do, it looks like a simple task but it took me a while
to make the damn thing function like a mini WordPad app.
Good luck.