Remove login blocking malware

I have bought a computer with Ubuntu pre-installed.

First, I saw strange message before distribution name in tty: "COPY NOT ACTIVATED".

When I attempted to login, I saw the following message:
Quote:

This GNU/Linux copy is not activated. You may be a victim of software counterfeiting. Logins are disabled until this issue is resolved.

root account has been disabled, so I had to use Live CD.

motd has been changed to say:

Quote:

This GNU/Linux copy is not activated. Error: "Could not resolve host: glatks.canonical.com" Current Key Server is: glatks.canonical.com Current product key is: 1485415b-a1ae-4d6c-ad90-e955f2361995

I have removed these files, but they reappeared one hour later. How do I get rid of this malware for good?