IE Error

A user has brought there machine to me with a problem with Internet Explorer.

When Dialed up to the internet, it will browse for around 1-2 minutes without a problem, then for no reason the browser will start reporting page cannot be displayed as if it is no longer connected to the Internet.

I belive either the user has changed something or that there is a virus/program running. I have tried Mcafee and Trend on line virus scanner but trend wont run and Mcafee runs very very very very slow on this machine.

Any ideas? I have serviced pached XP