https ok, http not

I am running xp, IE7 and have installed SP3. I access the corporate office via cisco vpn client. The home office does periodic updates, may have already installed SP3 but I doubt it. (Actually my wife's computer so I am never entirely sure what has or has not occured) My problem is that I can no longer access http pages only https pages, either when logged in thru vpn or not. Other computer on the network is working fine. I get the Internet Explorer cannot display the webpage error when attempting to log into yahoo, etc.
thanks for any help.