Knowledge, and anti-virus application. Stay updated with latest security patch,virus-spyware definition, DON'T EVER USE INTERNET EXPLORER, always use the limited account when doing web surfing on things you shouldn't be doing. Stay behind a router/firewall, and don't be stupid and click on anything you see (that's why I run in limited account when I do things...well things you shouldn't be doing). Scan for spyware atleast 3 times a month. Pretty much the best tool is knowledge, once you acquire it, you'll know.