The BEST thing you can do in that case is to use a different browser, anything other than Internet Explorer and Firefox. And make sure you keep everything up-to-date (especially Flash). Most of the anti-virus software out there is useless against the current spyware/malware threats that are the most common. They do nothing but waste PC resources. Using a different browser (Opera for instance) will keep any of the browser specific bugs (those that target a specific vulnerability in a particular browser) from affecting you, as the people writing these things generally target IE, and Firefox.

If you really want to stop it, download a Linux LiveCD, confront the porn viewer, and tell him to use the LiveCD to boot into linux to view his porn.