If you upgrade Rails to 2.2.2, you will find that the MYSQL adapter have been removed from the package. So you have to manually install it using command " gem install mysql".  but it is not over for works.  when you are running server and visit the home page, you will get error message on background "The bundled mysql.rb driver has been removed from Rails 2.2. Please install the mysql gem and try again: gem install mysql."   So surprise !  why ?
i search many pages with Google and find  the reason that the rails 2.2.2 is incompatible with mysql 5.x client. here is a solusion.  please followe the following steps to fix it:
1.  download older MySQL client library, for example one from InstantRails: http://instantrails.rubyforge.org/svn/trunk/InstantRails-win/InstantRails/mysql/bin/libmySQL.dll
2.  copy the downloaded file to C:\Ruby\bin (or wherever you installed Ruby)
3. restart MySQL server
that's it.
hope it will help you when you occur such problem.