Welcome to "The Quest for the Holy Desktop WMI Filter”, this is a global search for what you could call "The Perfect Desktop WMI Filter". A WMI filter which, by using WMI Query Language (WQL), should be able to spot DESKTOP computers only. It should be a general query - meaning it should be possible to use the filter in most Active Directory environments around the globe for Group Policy filtering.
So, what is a desktop really? Well, actually in this case we'll say it's the opposite of a laptop. Hmm, then what is a laptop? Easy enough: a computer with a battery! We've got the WMI filter for finding laptops already:
Select * from Win32_Battery - don't you just love the simplicity in this query?
This filter will make a computer with a battery respond back with "TRUE" (because the WMI class instance is present), meaning a GPO with this filter will apply to computers with batteries. Simple right? And you might think it's easy to just "turn it around" to find desktops, like:
Select * From Win32_Battery Where Availability != 2
or
Select
...