There are some cases where you need to identify a precise Windows 8 device (imagine a subscription that’s locked to a specific machine), in Windows Phone you could use:

Microsoft.Phone.Info.DeviceExtendedProperties.GetValue( "DeviceUniqueId" );

Unfortunately that was not available in WinRT but good new is that capability has been added into final release:

Windows.System.Profile.HardwareIdentification.GetPackageSpecificToken(null);

is all you need to get the identifier tied to a machine.

source: stackoverflow