Try this:
Select Distinct Top 1000000 tblassets.AssetID,
tblassets.AssetName,
tblassets.OScode,
tblassets.Userdomain,
tblassets.Username,
tblOperatingsystem.Caption,
tblOperatingsystem.InstallDate,
tblassets.Firstseen,
tblassets.Lastseen
From tblassets
Left Join tblOperatingsystem On tblassets.AssetID = tblOperatingsystem.AssetID
Order By tblOperatingsystem.InstallDate Desc