Microsoft has released a list of top 10 malware which infected Windows computers in Aug 2009.  This list is based on the data based on the Malicious Software Removal Tool.

The following table shows the breakdown by country/region.  US, China, and Brazil report the highest numbers of infected machines during the same time frame as the previous table.

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.  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.

As you look at this table you will see that the number of unique machines infected is lower than the total number of disinfections by MSRT.  There are several reasons for this including infections of multiple malware families on the same machine, multiple variants of the same family of malware found on the same machine and re-infections of the same machine over time.  MSRT is not a replacement for antivirus software with real-time protection from a known, trusted vendor. 

Source: Microsoft Malware Protection Center.

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