In the process of trying at install plugins on a new blog, I kept getting this very user unfriendly message

Incompatible archive PCLZIP_ERR_BAD_FORMAT (-10) : Invalid archive structure

I was trying install various plug-ins into the default installation of Wordpress 2.8 from my hosting company.

The fix was fairly simple. You need to replace the version of ../wp-includes/http.php and hey presto, all is well again.

The solution was posted in the following Wordpress trac ticket and gives the link for the correct version of http.php

http://core.trac.wordpress.org/ticket/10104