This article assumes that you are using Windows XP

Common Sense

First comes common sense. Without this, you have no chance of surviving in the hostile cyber world.

-Do not open unknown attatchments from emails.

-Do not download unknown files

Anti-Spyware Software

Anti-virus software is essential, without out it you may as well turn off the power now because you are at great risk of losing your data and privacy. I strongly reccomend you buy Norton Antivirus, which has been quite mainstream for a while. Website is here http://www.symantec.com/index.jsp and you can buy it at most stores.

If you don't want to pay for it, even though I reccomend you do, there are free options as well such as http://free.grisoft.com/

Firewall

You also need a firewall. Windows XP has an in-built firewall and you should update it if you can't find it. One thing to remember is to only keep one firewall on at a time. If you buy a norton firewall you can skip this part. Otherwise, read on.

One free option that has worked well for me is zone alarm, http://www.zonealarm.com/store/content/home.jsp

Firewalls can be annoying at times as they could block some of your software but most enable the user to unblock them.

Here is another page full of info on securing your pc,
http://antivirus.about.com/od/securitytips/Tips_for_Securing_your_PC.htm