You can make an Elgg into a subdirectory or subdomain, which ever you fancy but unzipping Elgg in the root directory of Wordpress is impossible. There can only be one type of index.php file in the root directory.

ok thanks,can u tell me the steps to make elgg and wordpress in parallel?  thanks

Karim, like Iionly said: "You can install Elgg and Wordpress in parallel on your server though. But it will be two separate sites then".  Mmm, Separate sites means, using diferent hosting/directory, diferent database or prefixed tables. That means are complete separated web application. I think the word "parallel" is misused referring the same server/hosting. I hope I could help.