The problem appeared with Zone alarm Extreme but, as it was confined to Browser Security, I believe it's a ForceField component issue.
I was trying to connect without success to http://www.tinyportal.co.uk.
After quite some struggling , I concluded that ZoneAlarm Extreme security software blocks the site after a few clicks and, a crazy thing, it blocks the modem router.
To be able to reconnect I had to reset the modem, wait for Internet connection to be reestablished, then a few clicks then the modem was blocked again.
It was a long story of searching, contacting my ISP provider, testing with three PCs, two desktops with XP, one with Vista, one with Ethernet two with wireless connection, plus blocked two other modems, two ISP providers. One may find more in
What setting affects www.tinyportal.co.uk (the site of the host has the same behavior) with ZoneAlarm Extreme.
Browser security -> Settings -> Advanced
Enable site status check <- Must be checked off
Blocking a site can happen for various reasons but blocking the modem router is an at least unexpected, if not undesirable, behavior for a security software.
ZoneAlarm support is informed, however they could not reproduce the issue.