I am installing AutoCAD 2012 LT, and I can install it fine. But when I run it for the first time I get a tick next to serial, and a cross next to product key. I have copied them exactly off the box.
Could you please send your serial number and product key through private message? (click on my arlinpega.bibat profile name and then "Send this user a private message") This is so that I may check further and see why the product is not letting you use the serial number.