Windows users based in the United States are the most likely to benefit from Microsoft's malicious software removal tool, which has removed malware from nearly 2.2 million US machines, more than the other nine top countries combined.

Over the same period, the MSRT has disinfected 383,378 machines in China, 282,152 in Brazil, 278,207 in the UK, and 262,539 in Korea, according to statistics Microsoft published here: http://blogs.technet.com/mmpc/archive/2009/08/27/msrt-august-top-detection-reports.aspx
In all, 2.18 million US-based machines were cleaned, compared with 1.87 million machines based in the other countries contained on the top-10 list.

"The US is at the top of this list as it is by default the top target for most of the malicious code out there," Marian Radu and Scott Wu, two members of the Microsoft Malware Protection Center wrote. "China and Brazil are actually a totally different story. While China is a top target for online games password stealers and the black market associated with it, Brazil is a prime goal for another breed of password stealers: those targeting bank accounts. Given these locations, it should come as no surprise that the top prevalent threats are what they are."

In August, Microsoft added a new trojan called Win32/FakeRean to its malware hit list. In the first two weeks the rogue anti-virus program was targeted, it was removed from 162,328 machines. A family of worms known as Win32/Taterf ranked No. 1, with 463,000 PCs cleaned. The worms spread over mapped drives in order to steal login and account details for popular online games.

Win32/Renos, another rogue anti-virus program, and a data-stealing trojan known Win32/Alureon, ranked third and fourth, with 228,973 and 211,441 machines purged respectively.

(Register)