Reference event logs and their analysis.
As said before, it depends how much you value your data.
There are syslog analysers available but they can cost more than a PC if you intend to use Event correlation (filter out the buls**t)
Physical analogy time again
...
Building higher walls is not the way to go.
If you have enougth detectors you can leave your front door open, you will catch the thief as soon as he enters
For the average cyber surfer I spose the latest freeware protection is a reasonable basic step safeguard to take. ok mr OZ?