Why can I connect to Internet with XP and not Vista

I am connecting to the net with an ADSL modem. Now, I have one PC that has XP installed and it connects every time and it does not drop the connection.
I have another Laptop with Vista and it is hell to connect with the same modem. I try wireless, cable, I switch off the other PC (no address conflict). At the beginning of the day it takes a lot of attempts to connect and then later it often drops the connection. Every time I access the modem it does connect to it. Sometimes I even get the icon at the bottom of Vista saying that there is Internet connection but I cannot access it. I usually get a message saying that the connection has timed out when trying to open web pages and downloading e-mail.

So I know that the broadband service is working and that the modem is connecting to the Internet and to the Vista laptop. It was connecting fine before. I made some changes to the Laptop to try and make it faster and it could have been this.

Can anyone give me some clues as to where to look? TCP/IP, etc?
Thanks in anticipation.

Does Vista have a system restore? I would hope it does, so use that to return the setup to the way it was before you made the changes. Then slowly introduce the changes checking after each one that connectivity is still there.

what sort of modem - make and model ?

I think I cracked it. I saw a microsoft site that talked about Vista having trouble connecting behind a firewall. I uninstalled the Firewall application as it was playing up and the Internet connection worked straight away.
Now I need to re-install it

Thank you