Help with SQL Server and XML columns
Try this:

SELECT
    x.MonitorId,
    x.configXml.value('(IntervalSeconds/text())[1]','INT') CheckInterval,
    x.configXml.value('(NumSamples/text())[1]','INT') SamplesBeforeDown,
    x.configXml.value('(SystemDriveWarningThreshold/text())[1]','INT') SystemDriveWarningThreshold,
    x.configXml.value('(CounterName/text())[1]','VARCHAR(100)') Countertype
FROM 
    (
    SELECT 
        MonitorId, 
        CAST( ConfigurationXML AS XML ) AS configXml
    FROM [dbo].[monitor] mt
    WHERE MonitorId = '4960E39A-59C8-A2C2-99B1-59B73D73156F'
    ) x


Note I have changed a couple of things about your code: 1) always use the text() accessor for better performance against untyped XML and 2) I've specified better datatypes.

Let me know if that works for you.