Another windows xp solution for "Unable to load dynamic library 'php_pgsql.dll' - The specified module could not be found."
Add the path of the postgres binaries to the system search-path:
Right click on "My Computer", select "properties", "advanced"-tab, "Environment variables"-button, "System variables"-section, select "path" and click "Edit" and add the path to the postgres binaries at the end, in my case it was "C:\Program Files\PostgreSQL\8.4\bin" and then restart the computer (or at least restart the apache/php processes)
Please note that the system-path can have several values, each separated by a semicolon ";".