Answer:
There is no way to protect your computer 100%. However, there are many things you can do to make the chances on a threat to your computer as small as possible.
You should have reliable Anti-Virus and Firewall software. Software that does not protect you ob the background, but so called scan on demand-software is useless. Viruses and other nasty software will only be found when the damage is already done.
Also, be careful with what you install and download. You can improve your security by simply thinking before clicking somewhere.