See if this SQL returns what you're looking for. For testing purposes, go ahead and change 'win.Guid' to 'win.*' to view all relevant information.
--Find all Windows computers that have the same MAC address as a Mac OS X device
SELECT win.Guid
FROM (SELECT Guid, [MAC Address]
FROM vComputer
WHERE LOWER([System Type]) IN ('win32', 'win64')) win
JOIN (SELECT Guid, [MAC Address]
FROM vComputer
WHERE LOWER([System Type]) = ('mac')) mac
ON mac.[MAC Address] = win.[MAC Address]