Problem/Summary
You receive the following error message : Error: the XML response that was returned from the server is invalid when attempting to install sample data.

Resolution

1. Install Joomla without installing sample data.
2. Do not delete the Installation folder after installation is done.
3. Download the joomla.sql file from installation/sql/mysql
4. Use PHPmyAdmin to install this SQL file into your Database.

If this does not help you resolve the issue go through the following link for further assistance:

http://forum.joomla.org/viewtopic.php?f=429&t=265877