Wifi properties lost on next boot

My configuration:
Satellite Toshiba A60 with internal wifi ethernet controller (Xp Home Ed.
Sp2)
Router/Access Point Zyxel Adsl (active dhcp)
If I use my internal ethernet card connected with a cable to the router
everything works fine (Internet and Email).
If use my wifi ethernet card to join the router I can surf the web but I
can't access to email box due rejected password (Thunderbird says PASS
command error - Outlook Express asks to confirm account password) (Instead I
can read web mail).
After some tests I found under wifi properties the Association Panel (I use
italian Xp so I try english translation: "Network authentication" set to
"OPEN" and "Data Cryptography" set to "Disable").
I tried to change "Data Cryptography" from Disable to Wep (the only one
choice from menu) and now email account works fine through wifi connection
under OE or TB but...
But when I turn off my computer and then I restart this setup is lost and
"Data Cryptography" is reset to "Disable".
Do you think is a problem of my wifi card? Router? Xp? My mistake?
It is frustrating must set up the proprierties on every boot up!
Any ideas?
Thanks in advance
Best regards
J

J.f.k.