The number of intrusion threats from internet are so numerous that one of my colleagues claimed he read somewhere that the average time for Windows XP to be breached once connected straight to internet is 30 seconds. Not necessarily true, but not unbelievable either.
Here’s an article that gives help on how to secure your computer. Some are a bit geared towards corporate users with sensitive data, but most apply to majority of normal users. Like carrying laptop all the time, what if you have a monster 5kg laptop? You’d break your back carrying it :). But some are really easy to do, like there’s no reason to use Internet Explorer when Mozilla Firefox is not only more secure, but better to use anyway. I am sure that if you’ve been using Internet Explorer, and you run Ad-Aware software, it will detect lots of Spywares and Adwares.