Sorry
I have got it figured out:
SELECT
dbo.tblComputers.Computername,
dbo.tblComputers.Username,
dbo.tblComputers.Lastseen
FROM
dbo.tblComputers
WHERE
computername NOT IN (SELECT dbo.tblservices.computername FROM dbo.tblservices WHERE dbo.tblServices.Caption = 'SMS Agent Host')