When it comes to permissions Magento Commerce is very picky, especially as pear requires full write permissions.

The following permissions should be used during install:
find . -type d -exec chmod 777 {} ;
find . -type f -exec chmod 777 {} ;

This will reset all permissions after you are done and secure your setup again:
find . -type f -exec chmod 644 {} ;
find . -type d -exec chmod 755 {} ;
chmod o+w var var/.htaccess app/etc
chmod 550 pear
chmod -R o+w media