How can we avoid Trojan malware?

Seriously, For those who have not or yet contracted these Trojan Malwares:

 

1. Where do they come from - specific websites, downloads, updates, games, etc

2. Does any product currently protect against them

3. Is Norton and others working on this as a national or global response

4. Do you just not use the Internet - improbable

 

Any preventive ideas would be appreciated by all (I'm sure), rather than wait for it to happen.

 

Thanks