"Hello world" problems: spam, hacker or…?

Hi, the problem I experience in my website (with wordpress) is that there is a line saying “Hello World” instead of text and in the foot area what appears is “Powered by Hello world!”. But I hadn’t any plugin installed, I mean no plugin with such a “Hello world” line!
Spam, hacker (it is just a blog!) or what else? Could somebody help me?

thank you in advance