Thanks for the reply. I just tried with the latest CVS version and the
problem still persists:

"GNU Emacs 23.0.90.1 (i386-mingw-nt5.1.2600) of 2009-02-21 on T42"

On Sat, Feb 21, 2009 at 4:16 AM, Eli Zaretskii <eliz@gnu.org> wrote:

> > Date: Fri, 20 Feb 2009 13:13:01 -0800 (PST)
> > From: mj <mj54590@gmail.com>
> > Cc:
> >
> > I have been having this problem since I switched to Emacs 23 several
> > weeks ago. I'm using VM to read my mails. There seems to be a problem
> > in decode-coding-region when VM tries to decode a string. When VM
> > tries to decode a region or a string, it uses a temporary buffer and
> > basically runs the following lisp code:
> >
> > (apply 'decode-coding-region (point-min) (point-max)  'us-ascii nil)
> >
> > The original buffer  content would be something like this:
> >
> > B7040400-12
> > some text here
> >
> > after decode-coding-region is executed, the buffer content became:
> >
> > ^@7040450-12
> > some text here
> >
> > Where ^@ is actually binary code \0 (not ascii ^ and @). There is another
> instance
> > that a string was decoded and the result is  ^@ prefixed.
> >
> > I could not reproduce this with "Emacs -Q". But it always happens when
> > thsoe particular messages were processed by VM.
>
> Could you please see if the problem still persists in the current CVS?
> Your Emacs seems to be about a month old (Jan 29), and a couple of
> related bugs were fixed in coding.c since then.
>