Too many to count. My system, and my ISP, have intercepted over 100 attacks by the W32/Netsky.D@mm virus. Just this week.

Have a good antivirus program. Norton can be instructed to keep your definitions up to date automatically.

There are hacks out there that will chew you up if you are running Windows, and don't have all the latest OS fixes installed. The fixes can be done online, and if you have later versions of Windows, you can instruct your machine to do it automatically.

And of course, use a firewall.

Doing without this stuff is like swimming at night off the Farallon Islands with a dozen raw steaks tied to your ass.