Ok I'm completely lost.
Just before I reinstall everything with a different OS (Win XP Pro), I'll throw a last message in a bottle.
2 weeks ago I've built a PC for my dad (his 1st one):
AMD Barton 2500
ASUS A7N8X
512 MB Ram
HIS ATI 9550
HDD MAXTOR 120 GB
...
D-Link PCI Wireless card
D-Link G604T ADSL router / Switch 4 ports /Wireless access point
The system runs on Windows XP Home OEM that I bought with the PC. The service pack 2 was already included on the CD (not as an option, Win XP installs itself with SP 2 from start).
Since day one, my father can't access some particular sites ie
http://www.france2.fr, the AH BBS, and most annoyingly
http://www.microsoft.com. He can't download his mails from a mail server: he can connect, list the number of mails waiting, but the retrieval hangs (i've tried this manually with a telnet 110 on different pop3 servers: always the same).
The DNS is ok: I can ping the above sites and even if they don't respond I see that the URL is translated into an IP address.
I thought at first that the problem came from the wireless modem (with integrated firewall) but I brought an old laptop running Win98SE and equipped with a wireless card: not a single problem there (I can access every site, I can download mails...)
I deinstall Norton2004 from my dad's PC, set internet security to NULL, shut down the firewall, install Mozilla Firefox (without transferring IE's setup!)....IE and Firefox share the same problem...
Oh yes network connections between dad's pc and laptop work fine too.
What can I try now to avoid reinstalling everything with a copy of Win XP Pro (pre SP1) to see if it improves?
No tech wiz that I know has the slightest idea about what's happening...
Thanks a lot already for even the tiniest hint, idea or advice.