When I enable any Drupal modules on the site, it through error message

Hi,

When I enable any Drupal modules on the site it through following error message and site is not accessible after that:

“The website encountered an unexpected error. Please try again later”.

Following errors I found after hitting button and then look on the watchdog table.

1)Message Notice: Use of undefined constant user_access - assumed 'user_access' in field_indexer_menu() (line 16 of /home/drupal/ww/sites/all/modules/contrib/xmlsitemap/field_indexer/field_indexer.module).

2) MessageNotice: Undefined index: smtp_server in zendframework_init() (line 15 of /home/drupal/www/sites/all/modules/zendframework/zendframework.module)

3) Message Notice: Undefined index: webcasts in zendframework_init() (line 29 of /home/drupal/wwwsites/all/modules/zendframework/zendframework.module).

4) Message Zend_Db_Exception: Adapter parameters must be in an array or a Zend_Config object in Zend_Db::factory() (line 228 of /usr/local/zend/share/ZendFramework/library/Zend/Db.php).

Any help to figure out the issue greatly appreciated! I'm using Pressflow Drupal 6

Reagrds,
Kuriakose