Using XP SP2 i have NEVER heard of anyone getting an infection without stupid behavior: it is not enough to click a link on a web page: you must allow the install!
if clicking on a strange link and then allowing an even stranger install to take place is not WHELP behaviour we have very few systems that are imune to infection , because Murphy's law states that you cant make a system that is idiot-proof because idiots are very creative and any system that is idiot-proof will only be used by idiots.
please remove this myth of super infectious malware that infect your machine by merely installing windows on it. Windows get more machines infected because there are more machines with windows installed. Linux NEVER was virus-free and on corporate systems Linux Based viruses are more prevalent because on corporate systems the windows based protections are more effective and react faster than Linux based ones, since they have a larger number of machines to report the infection from and a larger knowledge base to counter the virus.