I'm starting to see what you're trying to do now.  Try this:
Select Top 1000000 tblAssets.AssetName,
  tblAssetCustom.AssetID As LanSweeperID,
  tblAssets.AssetID,
  tsysAssetTypes.AssetTypename,
  tsysAssetTypes.AssetTypeIcon10 As Icon,
  Case When tsysOS.OSname Is Not Null Then tsysOS.OSname Else 'Not Scanned' End As OS,
  tblAssets.Domain,
  tblAssets.FQDN,
  tblAssetCustom.Manufacturer As Manufacturer,
  tblAssetCustom.Model,
  tblADusers.Firstname,
  tblADusers.Lastname,
  tblADusers.Username,
  tblADusers.Name,
  tblADusers.Displayname,
  tblAssetCustom.Location,
  tblADusers.Office,
  tblAssetCustom.Serialnumber,
  tblAssetCustom.SystemSKU,
  tblOperatingsystem.InstallDate,
  tblAssetCustom.PurchaseDate,
  tblAssetCustom.Warrantydate,
  tblAssets.Firstseen,
  tblAssets.Lastseen,
  tblAssets.Scanserver,
  tblAssets.IPAddress
From tblAssets
  Left Outer Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Left Outer Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
  Left Outer Join tblOperatingsystem
    On tblAssets.AssetID = tblOperatingsystem.AssetID
  Left Outer Join tblComputersystem On tblAssets.AssetID = tblComputersystem.AssetID
  Left Outer Join tblADusers On tblAssets.Username = tblADusers.Username And
    tblAssets.Username = tblADusers.Username And tblAssets.Userdomain =
    tblADusers.Userdomain
  Left Outer Join tsysOS On tblAssets.OScode = tsysOS.OScode
Where tblAssets.Assettype = -1
Order By tblAssets.AssetName
This query is assuming you only want to return Windows assets.  If this is not the case, let me know.