I bought a legal copy of Crysis2 yesterday from the local computer games store and I installed and went to register the serial number, but instead the error message read invalid serial key!

Any help, please?

You made a new thread, even though I answered your question in the last one:

Try to change your password or retype your CD key.
If that doesn't work, contact EA support: https://help.ea.com/contact-us
They can help with CD key related problems