For installing Webzash you need to make sure that the Apache mod_rewrite is enabled otherwise you will get a "Internal Server Error". Another way is to delete the .htaccess file in the root folder of your webzash setup.
Below are instructions to enable mod_rewrite :
Apache requirements for mod_rewrite to work
The Apache httpd.conf file has the settings to enable URL rewriting.
Two settings are required.
1. mod_rewrite module is installed
Search file for "LoadModule rewrite_module" and un-comment that line (remove the #)
2. AllowOverride All
Search file for "AllowOverride None" and change it to "AllowOverride All"
3. Restart Apache
Restarting the Apache web server is required for the changes to take effect.