POP3/SMTP connections fail after 2nd WinXP logon session

Very strange networking problem for the past 6 months:

Running WinXP SP2. Outlook 2002 SP2 configured for POP access to local
ISP's mailserver. ISP uses unsecured POP and SMTP -- ports 110, 25
respectively, no SSL or authentication. (I think they control access from
their own network IP ranges and if you are outside their net, you must use
webmail.)
Problem: after a reboot, Outlook connects fine and sends/receives mail.
However, after the initial WinXP session is ended (I log off of WinXp and
either I or another household member logs back in) network connections to the
ISP's mail servers fail. This is evident in Outlook which logs the "unable
to connect" error as well as in Windows Telnet to the POP server using port
110, so this is not an Outlook issue. Outlook Express configured with the
same email account settings also fails to connect. But other WinXP machines
on the internal subnet connect fine through the same broadband gateway which
is performing NAT from a 192.168.0 subnet to the cable network! Also, on the
same affected WinXP machine, browser connections from both Mozilla and IE
work fine, as do other apps that require a network connection. Can also
telnet to other POP3 servers such as Yahoo or Google, but they use port 995
and/or secured POP/SMTP connections. A reboot of the machine restores port
110 connectivity with my ISP, but again, just for that logon session.
Running Norton AntiVirus 2005 with all updates. Its logs don't show any
deliberate connection blocking to this IP (Windows XP SP2 firewall is
replaced by NAV worm blocking). Haven't changed ISP mail settings in years.
Very clean WinXP machine with minimal apps loaded. If this were an ISP
networking issues, I would think all machines on the subnet would experience
the problem but they don't. Seems tied to this one machine. Can't correlate
the problem with any specific Windows Critical Updates. Have installed all
Critical Updates as recommended by Automatic Updates. Can't tell is this is
being blocked by the ISP's servers or locally on my WinXP machine. Could
this be some kind of reverse DNS issue? What is it about a reboot that
clears this up for the first logon session?

Thanks for any help,

Frustrated in Optimum Online land.