Internet connection behaving weird.

System spec. E Machines 3240.
P4. - 505.
512 mgb. ram.
Win. XP. Service Pack 2. Fully updated.
Zone Alarm Firewall. Fully updated.
Windows Defender. Fully Updated
AVG Anti Virus. Fully Updated.
I.S.P. Tiscali Broadband.
Sagem ADsl Modem conncted to telephone line.
Firefox default browser.

Last night my internet connection started to behave weirdly. On attempting to open fiefox browser there was a long delay before the "unable to connect to server" error message appeared. Thinking this to be a Tiscali server downtime problem I carried on working offline. When some two hours later I tried to connect - same error message but unable to close the "connecting to tiscali"
box by right clicking. Task manager shows application running and closes it after posting "program not responding". Have run spybot, ad-ware and found nothing suspicious. Can connect through control panel but not by opening browser from desktop. Weird - Ticali inform me that all services are running 100%. Now Zone Alarm has recently updated and could this be why this has stated to happen?
Des Perate.