questionswhat is the best low power, low noise windows…

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You actually might have hit on the best answer - a Mac mini, booted into Windows.
Using Boot Camp, built into every modern Mac, you can install and boot into Windows natively (no emulation, no virtualisation).
http://www.apple.com/support/bootcamp/
Supported versions of Windows:
Microsoft Windows 8 (64-bit)
Microsoft Windows 8 Pro (64-bit)
Microsoft Windows 7 Home Premium (64-bit)
Microsoft Windows 7 Professional (64-bit)
Microsoft Windows 7 Ultimate (64-bit)
So, if you get yourself a Mac mini and a copy of 64bit Windows of your choice, you will end up with a low-cost, low-power-consuming, quiet little Windows machine to run your Quickbooks and Filemaker servers on.
-=C=-

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Thanks for the answer! This is actually exactly what I did. It works great and uses about 1/4 the power of our old machine.

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A lower-cost alternative would be this machine, which is fanless and draws very little power. http://bit.ly/14SiqCT