When setting up additional user accounts (using Windows XP
Professional), I am subsequently unable to access the
internet. My ISDN dial up connects, but when opening
Internet Explorer or Outlook Express, an error message is
experienced saying server could not be found. In other
words, although the modem appears to have connected,
Internet Explorer and Mail can't connect.

Any idea what I'm doing wrong?

go back to additional user accounts go to your account called administrator
or goto help and support centre goto search type in isdn click green arrow
to search for it