Should be something like this:
SELECT TOP 100 PERCENT dbo.tblADComputers.OU, dbo.tblComputers.Computername, dbo.tblComputerSystemProduct.Name,
dbo.tblComputerSystemProduct.Vendor, dbo.tblComputerSystemProduct.IdentifyingNumber
FROM dbo.tblComputers LEFT OUTER JOIN
dbo.tblADComputers ON dbo.tblComputers.Computername = dbo.tblADComputers.Computername LEFT OUTER JOIN
dbo.tblComputerSystemProduct ON dbo.tblComputers.Computername = dbo.tblComputerSystemProduct.Computername
ORDER BY dbo.tblADComputers.OU, dbo.tblComputers.Computername