You can try this:
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetUnique,
tblAssetCustom.Model,
tblADusers.Displayname,
tblOperatingsystem.Caption,
tblAssetCustom.Warrantydate,
tblAssetCustom.Serialnumber
From tblADComputers
Inner Join tblAssets On tblAssets.AssetID = tblADComputers.AssetID
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Left Join tblADusers On tblAssets.Username = tblADusers.Username And
tblAssets.Userdomain = tblADusers.Userdomain
Inner Join tblOperatingsystem
On tblAssets.AssetID = tblOperatingsystem.AssetID
Order By tblAssets.AssetUnique