Problem (still) accessing internet across network

Thanks for the suggestion of specifying the IP address of
the ISP DNS servers, but it didn't solve the problem - I
wonder if I did the right thing. I set the DNS IP
addresses in the TCP/IP properties of the internet
connection on the desktop and of the network connections
on the two PCs. Was this correct?

Thanks for any further help.

Original problem:

I have a peer to peer wireless network between a laptop
and a desktop, both running XP. File and printer sharing
work (thanks to getting tips from this newsgroup!).

The desktop has an ISDN connection and works fine for
internet access. I want to use this connection on the
laptop across the network.

For some websites it works correctly from the laptop, but
for others the progress bar creeps up to 5 marks and then
hangs indefinitely, with the message 'opening' and the
url. Examples are news.bbc.co.uk which works, and
www.amazon.co.uk, antwrp.gsfc.nasa.gov/apod/astropix.html
and support.microsoft.com which don't.

Everything works OK if I connect the laptop directly to
the ISDN line. (I then have to delete the connection I
defined to make it use the network again).

I'm running Norton Internet Security, but I get the same
problem if I turn it off.