I guess to limit downloads to websites you trust only :)!! And don't download anything else.

My computer is too cluttered Anyways so that's probably a good thing :)

Use of proper firewall like "Zone alarm" will prevent any request from your computer to attacker server, and hence you will be notified what is requested.  So even you end up with downloading any bad files / software e/t/c. You will know by your firewall alert and best part it will be blocked automatically. So even the software is running in your computer it will do no harm and later on you can uninstall suspicious software(s). 

Well a person needs to be careful regarding these things themselves......Little knowledge is important ....but how will they this knowledge and why?

Use something like our http://www.itsystemsol.com.au/index.php/msp/endpoint-security which will prevent problems happening at all levels, virus, firewall and application control.

A good anti-virus software is important.

Avast is a free antivirus which prevents you from going to dodgy web sites or receiving virus/trojan attachments.

Endpoint protection (see above) is an inexpensive enterprise solution. (PS: I do not know Shaun or IT Systems Solution but I know Endpoint Security.)

Note: Firewalls are mandatory and a must. However, the new breed of malware can tunnel thru firewalls. So a good security system has to have several layers. I no longer use "Zone Alarm". I use Avast for home computers but will use Norton or Endpoint for business computers.

Usually more than one anti malware program is better than a single program.

I use Panda Security and Malwarebytes running all the time - dangerous websites are blocked automatically, no malware gets through, and if anything dodgy is opened, it is automatically quarantined.