The best defense is to keep any form of malware from ever getting to your computer. Here are some steps you should be taking (geared toward Windows users):
- Keep your firewall turned on
- Install the automatic updates - many are security updates that combat the latest threats
- Never click on links in emails unless you know it's a trusted source, and be extra careful about downloading attachments
- Use strong passwords, check their strength here: Password Strength.
- Scan, scan, and scan again. There are free products that do a great job keeping your system free of malware, some are: AVG Free, MalwareBytes Free, as well as Microsoft Security Essentials. I'd recommend running multiple scanners until you find one you're comfortable with that doesn't compromise system performance.
- Minimize spam with a solid email client. GMail is tough to beat. GMail makes available advanced sign-in security, which will go a long ways toward eliminating those mass spam emails to everyone in your contact list.
There are other steps you can take, enabling User Account Control is one, but we'll keep it simple for now. Mac users may chuckle and say, "get a Mac", but the Mac has it's own share of malware as well.
What do you do to keep your system free of malware?