If you formatted your old PC, CCD will not work on the same key anymore (source: Kivlov). You have to obtain a new key by following these steps:
1. After installation of the program is finished, run the installed game with administrator privileges (right click on the game's shortcut and choose "Run as administrator in the context menu).
2. Copy the product code from the game startup window.
3. Follow this link -
www.activate.citycardriving.com/
4. On the web page you've just opened you should enter: your serial number, the product code you have just copied, and your e-mail.
The activation key will be displayed on the final page, after you'll click the "Activate" button.
5. Enter your activation key into the box in the game startup window and click "Registration" button.