Not sure I am understanding your question, but you get your username
and password from your mail provider (Comcast in this case).

If you have the correct username and password, but it is not working
in Windows Mail, copy the error message and post it here.

Comcast has to assign your username before you can use it, and keeps a
record of what it is.  They can also change your password to something
new if you have forgotten it, but if you have them do this, you should then
change it again to something they don't know.

Note that Comcast is changing what port numbers it requires users to
use, but not doing a good job telling users about this.  You're soon likely
to need this information:

<http://help.comcast.net/content/faq/How-to-configure-Windows-Mail-for-Vista-to-u
se-port-587-for-sending-email>