One of the Internet security companies (McAfee) that monitors hacker activity (by compiling data on attacks their clients have suffered), estimates that more computers are being turned into zombies in China, than in the United States. At least during the last three months of 2009. Both countries have about 12 million zombie computers.
Many of 1.5 billion computers in the world are infected with secret programs that enable criminals, or intelligence agencies, to control these PCs, turning them into "zombies". These captive computers are organized into "botnets" of thousands, or even a million or more, PCs that do the bidding of their controllers. The most common use of botnets is transmitting spam, and secret programs that create more zombies, or steal information (government secrets, or your banking information.)
Most owners of zombiefied computers didn't even realize their PCs had been taken over. Some with heavily infected machines, do notice that the malware slows down the PC, and there have been cases where the user just went out and bought a new computer. Usually, reformatting the hard drive and reinstalling your software works, and is a lot cheaper. But most computer users today don't know how to reformat a hard drive, or even get someone to do it for them.
Meanwhile, computer security companies equip their anti-virus software with the ability to remove the secret software that turns PCs into zombies. The most successful of these efforts has been Microsoft's, which automatically updates its operating system and its security software, and removes secret hacker software in the process. This effort has set over ten million zombie computers free from their control software.
Chinese PCs are particularly vulnerable to getting zombified because most of them use stolen operating system software, and have less security software installed. Thus while the Chinese government is the most aggressive user of hackers, and zombie computers, for espionage purposes, the 200 million PCs in China are more vulnerable to such attacks than those in Western nations.