From a military perspective, this is bad because the United States economy, and military, is very dependent on the Internet. With all those zombies within the United States, foreign cyberwarriors could use a large zombie army (a few hundred thousand PCs), to shut down much of the Internet in the United States, without shutting down the entire Internet. More worrisome is the possibility that a lot of military PCs are infected with malware. These PCs are watched more carefully for spyware and malware infection, but its so damn easy for that stuff to sneak onto a PC. And the most dangerous stuff is usually not noticed by users. All of this is a set-up for someone, maybe everyone, to get hit with a digital Pearl Harbor.
The United States just got reminded of how vulnerable it is to a cyberwar attack, because of a recent survey of all PC users. Turns out that 43 percent of Americas 135 million PC users knew that they had been hit with spyware (software secretly placed on your PC for the purposes to tracking your web use, all to better send you ads). The evil twin of spyware is malware (evil software), that does stuff like take control of your PC on command of some distant cyberwarrior. The most common malware are zombie programs (that, in effect, turn your PC into a zombie, controlled by that evil cyberwarrior or criminal). Spyware is relatively easy to spot, because it does stuff like make ads pop up when you are not online, or not using your browser. There are also a lot of security programs out there constantly searching for spyware. Malware, because it is often used for outright illegal purposes, hides itself better and is more difficult to uncover. But physical examinations of a random sample of PCs have indicated that there are at least several million PCs out there infected with malware, and thus ready to be turned into zombies.