Dell major messup, won't accept returns even though it is their fault

I ordered a Dell Vostro 1520 which Rama Rao in Dell Chat informed was eligible for Windows 7 upgrade.

When it arrived it wouldn't register for the upgrade. I ordered the next model up which was eligible for the upgrade and processed a return on the original one. The Dell website suggests that a machine can be returned within 7 business days of delivery but Dell refused to do so.

Dell wrote back that they didn't accept returns. Then I had to cancel the second order.

I did so and the lady confirmed the order had been cancelled (24710655). However the next day Dell wrote that they had shipped the order despite it being cancelled.

Now I am back to square one and they will not accept a return on the second one. I don't want two Dell machines and I don't want to pay for two machines.

Daniel