Shooting in the dark here, but try this...
Select Top 1000000 Coalesce(tsysOS.Image, tsysAssetTypes.AssetTypeIcon10) As
icon,
tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Domain,
tblAssets.Username,
tblAssets.Userdomain,
tblAssets.IPAddress,
tblAssets.Firstseen,
tblAssets.Lastseen,
tblAssets.Lasttried,
tsysOS.OSname
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysOS On tsysOS.OScode = tblAssets.OScode
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Where
tblAssets.AssetID Not In (Select Top 1000000 tblCertificateLocations.AssetID
From tblCertificates Inner Join tblCertificateLocations On
tblCertificates.CertificateID = tblCertificateLocations.CertificateID
Where tblCertificates.Serial = '18DAD19E267DE8BB4A2158CDCC6B3B4A')
And tblAssetCustom.State = 1
Order By tblAssets.Domain,
tblAssets.AssetName