Displaying the Drupal 6 webform in a block
To display webform in a block you need to install extra module. Webform block modules provide the functionality to display form in form of block.
Download and install the webform block module from http://drupal.org/project/webformblock
Displaying the webform in a block for Drupal 7
To get the webform as block:
Create your webform in a node.