Well, we all know (or should know), that there is no 100% security! (of course people might try to take this out of context and apply this to tiny subsets of an overall problem and claim it can be 100% secure but its a futile attempt as you have to look at security in a holistic manner otherwise you will run the risk of false sense of security) Anyway, so there is no 100% security. So what you have to do is to utilise a layered architecture, eg: use different Security systems in case one of them fails. The question is which ones and where do you stop?
Which ones: it has to be synergestic and avoid duplication unless you are looking redundancy (but sometimes its not a good idea: eg Firewall, having two of the same is a no-no.)
Where do you stop: Its all about what you are trying to protect and how much you are prepared to spend to protect whatever you are trying to protect. The higher the value that you are trying to protect the more you spend in security. “Value of the Asset” and “Cost of Security” is proportional.
Value of the asset
| x
| x
| x
| x
| x
|———————–Cost of Security
But of course, for your PC Security the Cost of Security is ZERO thanks to Comodo 
Melih