633 and 797 error messages

A friend of mine has an HP Pavilion Slimline s7712n Desktop PC. He is
running Vista Home Premium and uses an Agere Systems US 2.0 Soft Modem
(on
COM 3) for a dial-up connection to his ISP, Net Zero.

For quite a while now, he has been getting 633 and 797 error messages.
The first one says "port already open or in use" and the second one says
"a connection to the remote computer could not be established / modem
was not found or busy."

I had a brief look at it yesterday (I told him I'd need to return to
spend more time with it). This is what I saw:

There is no modem in Device Manager. There is one exclamation mark in
Device Manager next to Microsoft 6to4 Adapter. My guess is this is the
source of his problems. I'd imagine it's totally unnecessary for him as
well. It appears he has the most recent driver for his modem (2.1.77.0).

My tentative plan for my next visit will be to remove the 6to4 adapter
from Device Manager. How is this best done? Through Device Manager? Or
should I remove IPv6 (there are directions for this on the Microsoft
site*)? Or should I do both?

I'm thinking I need to uninstall/reinstall both Dial-Up Networking and
the modem. Do I need to unplug/replug the modem? What is the best way to
do this?

Am I on the right track? Thanks in advance.

*
http://www.microsoft.com/resources/d....mspx?mfr=true

--
Dave