Watch the indicator light on your modem. If its starts to flash or blink, someone may be trying to send you unwanted software.
Surf fast. If you do not linger on any one page for more than a few seconds, you can reduce your risk of drive-by downloads
Make sure that the security setting for your browser is high enough to detect unauthorized downloads (for eg: atleast the medium setting in Internet Explorer). Also make sure that you have installed the most recent version of the browser software.