try this:
Select top 1000000 tblCompCustom.Department, tblComputerSystemProduct.Vendor,
tblComputerSystemProduct.Name, Count(tblComputers.Computername) As Total
From tblComputers Left Join
tblCompCustom On tblComputers.Computername = tblCompCustom.Computername
Inner Join
tblComputerSystemProduct On tblComputers.Computername =
tblComputerSystemProduct.Computername
Group By tblCompCustom.Department, tblComputerSystemProduct.Vendor,
tblComputerSystemProduct.Name
Order By tblCompCustom.Department, tblComputerSystemProduct.Vendor,
tblComputerSystemProduct.Name