try running this in the database manager, if you get results, then your query is solid and you need to look at other reasons why this alert isn't firing
SELECT Caption, VolumePercentUsed FROM Volumes WHERE ( (Volumes.VolumeType = 'Fixed Disk') AND (100-NullIf(VolumePercentUsed,-2) < 60) AND (Nodes.WindowsServersv3 = 1) )