"Microsoft’s latest Security Intelligence Report, available for download here
, provides information on Windows, application and web security in the year 2010. The report reveals that Windows XP SP3 has an average infection rate of 15.9 computers per thousand. Vista halves the figure to 7.5 and Windows 7 pushes it to just 3.8. The 64-bit editions of Vista and Windows 7 knockdown the infection rate even more with 5.3 and 2.5 infections per thousand computers respectively. The figures mean that the 64-bit edition of Windows 7 is 6 times less likely to be infected than Windows XP."
This was found at:- Windows 7 malware infection rate significantly lower than Windows XP | WinRumors