Activation under a proxyserver - Code Error 2146697 please help

I’m having troubles trying to activate cpf.
Situation :

Win Xp Pro SP2
domain member
Trend Micro Office Scan
receiving internet connection from a cascading Proxy Server that needs authentication
proxyserver only allows port 80
file hosts is clean
IE 6 Sp1, browser settings ok
Which port is used by the activation process?
Updater asks user and password to the proxy server,and everything is ok
Activation process does not.

Any help will be very appreciated!
Thank you