For those of you wanting to filter your GPO's by OS here are the filters we use. The filter works against the root\CIMV2 namespace
Windows 8
Select * from Win32_OperatingSystem Where Version like "6.2%" and ProductType = "1"
Windows 7
Select * from Win32_OperatingSystem Where Version like "6.1%" and ProductType = "1"
Windows XP
Select * from Win32_OperatingSystem Where Version like "5.1%" and ProductType = "1"