try this:
Select tblComputers.Computername, tblComputers.ComputerUnique,
tblComputers.Domain, tblComputers.Userdomain, tblComputers.Username,
tblADusers.Displayname, tblADusers.email, tblComputers.OScode, tsysOS.OSname
From tblComputers Left Join
tblADusers On tblADusers.Username = tblComputers.Username And
tblADusers.Userdomain = tblComputers.Userdomain Inner Join
tsysOS On tsysOS.OScode = tblComputers.OScode
Where tsysOS.OSname = 'Win 7'