try this:
Select Distinct dbo.tblComputers.Computername As Computer,
dbo.tblOperatingsystem.Description, dbo.tblOperatingsystem.Caption,
dbo.tblComputers.Lastseen As [Last seen]
From dbo.tblComputers Inner Join
dbo.tblComputersystem On (dbo.tblComputers.Computername =
dbo.tblComputersystem.Computername) Inner Join
dbo.TsysLastscan On (dbo.tblComputers.Computername =
dbo.TsysLastscan.Computername) Left Outer Join
dbo.tblOperatingsystem On (dbo.tblComputers.Computername =
dbo.tblOperatingsystem.Computername) Left Outer Join
dbo.tblSoftware On (dbo.tblComputers.Computername =
dbo.tblSoftware.ComputerName) Left Join
tblCompCustom On dbo.tblComputers.Computername = tblCompCustom.Computername
Where dbo.tblComputers.Computername In (Select TsysLastscan_1.Computername
From dbo.TsysLastscan TsysLastscan_1
Where TsysLastscan_1.CFGname = 'Software') And tblComputersystem.Domainrole <
'2' And
dbo.tblComputers.Computername Not In (Select tblSoftware_1.ComputerName
From dbo.tblSoftware tblSoftware_1
Where
tblSoftware_1.softwareName Like
'Compatibility Pack for the 2007 Office
system') And
IsNull(tblCompCustom.State, 1) <> 2