If you use Microsoft Windows on a public network your computer may already be under the control of somebody else - and for once I'm not talking about Microsoft here. I am referring to nefarious malware authors, spammers, script kiddies, etc. If you download and install software from the internet your computer most likely is so compromised.
The odds are much higher than you probably imagine.
And, if your system has been compromised, you are probably part of a botnet. From the linked article: "It has been estimated that up to one quarter of all personal computers connected to the internet are part of a botnet."
If your computer is part of a botnet, it can be used to attack servers on the internet, including commercial and military targets.
Now, here is where it gets interesting. "If the United States found itself under a major cyberattack aimed at undermining the nation’s critical information infrastructure, the Department of Defense is prepared, based on the authority of the president, to launch a cyber counterattack or an actual bombing of an attack source."
Yes sir - use Microsoft Windows, and the US DOD will drop a bomb on your house. You have been warned.