2 Million PC that runs Windows are Infected

Out of the 4 Million PC runing Windows , 50% are infected by a backdoor Trojan. "Attackers, with financial gain in mind, are clearly concentrating a significant amount of development focus on this category of malware," Microsoft said in the report. A computer compromised by such a Trojan horse, popularly referred to as a zombie PC, can be used by miscreants in a network of bots, or "botnet," to relay spam and launch cyberattacks. Additionally, hackers often steal the victim's data and install spyware and adware on PCs, to earn a kickback from the spyware or adware maker.

Rootkits, which make system changes to hide another piece of possibly malicious software, remain an uncommon threat. There has been a 50 percent reduction in this kind of attack against computers running Windows during the past six months, Microsoft said.

Microsoft introduced the Windows Malicious Software Removal Tool in January last year. An updated version of the program ships monthly with Microsoft's security updates. The tool aims to identify and remove prevalent malicious software from PCs.