I trust that these computers are XP SP3 and have IE8 installed and totally up to date? The Win 7 one should be SP1 with IE9 installed. Whether or not you use IE is irrelevant as many processes do, including McAfee so may as well make sure.
I'm assuming you've already tried right-clicking the network icon by the clock and selected Diagnose and Repair, or whatever (varies by system)?
In Device Manager is the Network Adapter up and running?
Click Web and Email Protection
Click 'Restore Defaults' and close it.
See if that helps.
Sorry to take so long to answer you but I was hoping a technician would step in here as they are more knowledgeable that I.
They are available by phone should you need them and it's free of charge. See the link for Technical Support under Useful Links at top of this page.