I've installed everything from zero (Windows,
Eclipse, Ant, Ant Contrib). That problem disappeared. As a comment, I've used installation folders without spaces (C:\Java, C:\Eclipse, etc.) and I've added the Bin folders of JRE, JDK, and Ant to PATH environment variable.
Then I had some issues with Java memory heap, but the problems were solved with this article:
http://javahowto.blogspot.com/2006/06/fix-javac-java-lang-outofmemoryerror.html
Now I have a new problem with jarsigner, but I will make a new post for that.
Thanks Tim.
Regards,
Juan