1) A good anti-virus program.
2) A good firewall.
3) Stay away from suspicious sites.
4) Don't open suspicious emails.
5) Don't click on anything if you don't know what it is.
6) Keep your OS and software updated.

I don't even remember the last time I had a virus, it's been so long, so I know for a fact this works.

i only use AVG lol