You did in fact receive a free 3 year termed Student License of AutoCAD 2012. The "thank you for purchasing" statement is most likely a standard message that is displayed after all successful installations / activations regardless of license type.
Please proceed with the re-installation, accept as a solution if this fully answers your inquiry and let us know if you run into any issues.
I am very happy that response, because I need it necessary for my study. Well, due to my currently less powerful computer, you'll probably need to download an older version of AutoCAD. Thanks for your reply