* Place the htmlpurifier folder in your drupal modules directory.

* Download HTML Purifier from http://htmlpurifier.org/

* Extract the "library" folder and place it inside your modules/htmlpurifier
  directory.

* Go to Administer > Site building > Modules and enable this module

* You can now create a new input format or add the HTML Purifier to an 
  existing input format. It is recommended that you place HTML Purifier as 
  the last filter in the input format. Reorder the filters if necessary.