Internet Problem - (Unable to Display Page)

My son's computer is experiencing a similar problem - it connects to the internet for maybe 1 or 2 sites and then gets an error saying that "Unable to display page ". If I log off and then log back on then he has internet access for 1 or 2 sites and then down again.

His computer is running Windows XP Home. He is connected to the internet through a routier that has another two computers connected to it and the other two are fine no problem so it's obviously something on his computer.

Unfortunately he is a download freak and I'm afraid that perhaps he has downloaded some sort of virus or worm but I can't access the internet long enough to check it through Microsoft. The Spy Sweeper program he has doesn't seem to pick up anything and the anti-virus is working and also not picking up anything.