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#21
To get invincible, you need more than to unplug it and put it in the closet. This still leaves the possibility of someone gaining access to the physical box and installing malware. You get closer to invincible if you enclose the system in a thick lead box, then encase the box in several feet of alternating layers of concrete, lead and armor plating. Take the resulting room sized box and launch it into deep space. This ensures that the number of potential hackers with access to the box are as limited as humanly possible.
However, all of the advice offered in this thread is good... proper protective measures (software/firewall), and avoiding risky activities get you as secure as you are going to get for a usable system.