Vista - Modem / Internet / RealConnect Agent problem

Good afternoon

I bought a laptop (Toshiba Satellite) last week with Vista Home
Premium pre-installed. The first thing I did when I got it was set up
a dial-up internet connection (the only form of internet connection
available to me at that time) in order to download the software to do
an Instant Upgrade to Vista Ultimate. I did this successfully.
However, since then I can't get an internet connection any more. When
I try to start the dial-up connection the modem appears to work, the
usual phone line sounds occur, the computer tells me that my ISP
username and password are accepted and I am logged on to the network,
yet when I then try to open Internet Explorer I get:
"you are working offline"
"you are not connected to the internet" and/or
"Microsoft RealConnect Agent is not responding"

Help anyone? Thank you.

Peter.