Problem with PostgreSQL and Ruby on Windows

When I try

gem install pg

I get this error

C:/Ruby192/bin/ruby.exe extconf.rb checking for pg_config... yes Using config values from C:\Program Files\PostgreSQL\9.0\bin/pg_config.exe * extconf.rb failed * Could not create Makefile due to some reason, probably lack of necessary libraries and/or headers. Check the mkmf.log file for more details. You may need configuration options.

Provided configuration options: --with-opt-dir --without-opt-dir --with-opt-include --without-opt-include=${opt-dir}/include --with-opt-lib --without-opt-lib=${opt-dir}/lib --with-make-prog --without-make-prog --srcdir=. --curdir --ruby=C:/Ruby192/bin/ruby --with-pg --without-pg --with-pg-dir --without-pg-dir --with-pg-include --without-pg-include=${pg-dir}/include --with-pg-lib --without-pg-lib=${pg-dir}/lib --with-pg-config --without-pg-config --with-pg_config --without-pg_config extconf.rb:13:in `': No such file or directory - C:\Program Files\PostgreSQL\9.0\bin/pg_config.exe --includedir (Errno::ENOENT) from extconf.rb:13:in'

Any advice?