Connection Refused Periodically

In the past few days something strange has been happening with the Internet
connection on my laptop. When I open a browser and go to a few sites, after
a few minutes I get the message "Connection refused" when trying to go to
another site. Note the following:

1) This happens on my laptop computer, but not my desktop computer.

2) The laptop connects to the Internet through a DSL connection via a
wireless connection to a router.

3) When this problem occurs, the laptop has no problem with its connection
to the router, as it's able to connect to the other computer without any
problem.

4) Also, when this problem occurs, the laptop has no problem sending or
receiving e-mail.

5) Also, when this problem occurs, the desktop computer (the other computer
connected to the router and sharing the Internet connection) has no problem
browsing the web or sending or receiving mail.

6) This problem usually happens while the laptop is browsing using Firefox,
as that's the default Internet browser. However, when it happens, the laptop
gets the same message using Internet Explorer as well.

7) The problem usually can only be resolved by rebooting the laptop. When
the laptop reboots, it browses fine using Firefox or IE. In the past,
closing and re-opening the browser did not resolve the problem. However,
tonight, for some reason, when I closed and re-opened the browser, I was
able to browse again.

8) The laptop is running Norton Internet Security 2006. Virus scans have not
turned up anything.

9) Disabling Norton Internet Security (in case it was causing the problem)
did not resolve the problem.

The OS is XP SP-2.

Any ideas about what might be causing this would be appreciated. Again, this
has only been happening the past few days. The laptop has been working fine
in this configuration for well over a year.

Thanks!

Neil