Internet Explorer Browser Error

My girlfriends computer’s internet explorer won’t load a page. We know that it is connected to the internet, that’s not the problem. The problem is when she tries to open up her browser, the browser won’t load anything. It comes up with a normal loading error when it cannot find a site, but it does this for every site she tries to load. I think she has viruses and spyware as her computer is really slow sometimes for no reason. Also, she has had to reinstall her Microsoft Word and it still doesn’t work. She has no virus protection other than what is on Vista (Defender and the firewall). Any ideas on what is going on and how to fix it?