[Narrew]

What Thermodyne and Suicide Junkie said. If it was just one one web site that was bumping you, they may have been hacked and re directed.

But I would add to what the others have mentioned is that you set up a User Account for your roomie, XP is good for that (with Win 98 you could just cancel the log in and still get local access, but XP or Win 2K you can lock it down) and you can restrict his ability to modify your machine.