Try this:
Select Top 1000000
tsysOS.Image As icon,
tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Domain,
tblAssets.FQDN,
tblAssets.IPAddress,
tblADComputers.OU,
tblAssets.Firstseen,
tblAssets.Lastseen,
tblSoftwareUni.softwareName,
tblSoftware.softwareVersion,
tblSoftwareUni.SoftwarePublisher,
tblSoftware.Lastchanged
From
tblAssets
Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tblSoftware On tblAssets.AssetID = tblSoftware.AssetID
Inner Join tblSoftwareUni On tblSoftware.softID = tblSoftwareUni.SoftID
Left Outer Join tblADComputers On tblAssets.AssetID = tblADComputers.AssetID
Inner Join (SELECT
tblSoftware.AssetID
FROM
tblSoftware
INNER JOIN tblSoftwareUni ON tblSoftwareUni.SoftID = tblSoftware.softID
WHERE
tblSoftwareUni.softwareName Like N'INSITE%') AS HasInsite ON HasInsite.AssetID = tblAssets.AssetID
Where
tblAssetCustom.State = 1
The sub-SELECT produces a list of assets that have the INSITE software installed.
The INNER JOIN limits the main SELECT to only those assets in the sub-SELECT list.
The main SELECT pulls a list of all software for those assets that are in the sub-SELECT list.