XP will continue to lay claim to the title of the leanest 64-bit NT kernel-based Windows platform (sorry, Windows 2000, you're 32-bit only), which is evidenced by its superior execution efficiency. If you take the raw transaction times for the database and workflow tasks, then factor them against the average processor utilization for these same workloads, you see that Windows XP consumes roughly 7.2 and 40.7 billion CPU cycles, respectively, to complete a single pass of the database and messaging workflow transaction loops on our quad-core test bed. By contrast, Windows Vista takes 10.4 and 51.6 billion cycles for each workload, while Windows 7 consumes 10.9 and 48.4 billion cycles. Translation: On quad-core, the newer operating systems are at least 40 percent less efficient than XP in the database tasks and roughly 20 percent less efficient in the workflow tasks.
In the end, it all comes down to the complexity of the execution path. With its simpler legacy kernel devoid of DRM hooks and other performance-sapping baggage, Windows XP provides a cleaner code path for the workloads to navigate as they execute. This, in turn, translates into better overall performance with lower consumption of CPU cycles. The flip side is that these performance advantages are likely limited to the current generation of quad-core systems, and perhaps tomorrow's eight-core CPUs, and may quickly evaporate as new system designers seek to incorporate increasing degrees of parallelism into their products....
Clearly, the optimizations made to the Vista kernel -- both in its original incarnation and in its updated Windows 7 variant -- are having an impact even at the quad-core level. However, better scalability still isn't enough to offset Windows XP's huge performance edge on today's hardware. In fact, it won't be until after Windows 7 has been replaced by the next Windows that the fruit of Microsoft's multicore optimization labors will be fully realized. Then, as we boot our 32- or 64-core netbooks, we can all smile as Microsoft's foresight and perseverance finally start to pay off.
open source 
! Vista makes it easier for me as it is resistant to most of the USB virii going arnd but the h/w requirements are a strict no.
