I'm trying to authenticate to our Sophos filtering system with IE8 on a netbook with Windows 7 starter. I've done this before many times with other netbooks and I'm almost 100% sure at least one of them have been Starter also. I have no explanation why these netbooks fail.

I've checked (and disabled the windows firewall). I've reset the setting in Internet Options. I've also installed Google Chrome and it also fails to authenticate.

I've gone to another machine (off the network) to test if the authentication works and it does.

Any ideas on why these machines aren't authenticating, what setting could I be missing?