Or you could just install the free AVG, the cost is zero (if money is what worries you), the impact on performance is negligible (I do photo and video processing, the antivirus doesn't seem to make Vista slower than it already is), and it's definitely better than nothing (caught a few cockroaches trying to sneak into my system already).
But what do I know, my laptop is running Ubuntu, my primary desktop has always been Linux since 1997.
But what do I know, my laptop is running Ubuntu, my primary desktop has always been Linux since 1997.