The documenation states in http://www.php.net/manual/en/install.windows.php about php.ini location on Windows:

Copy your chosen ini file (see below) to your '%WINDOWS%' directory on Windows 9x/Me or to your '%SYSTEMROOT%' directory under Windows NT/2000/XP and rename it to php.ini. Your '%WINDOWS%' or '%SYSTEMROOT%' directory is typically: 

[...]

However, php.ini can also be placed in to the directory where php is installed. This is more error save, because one does never have an old php.ini file dumpling in the windows directory. Additionally it's possible to run multiple php installations with different ini settings on the same system if php.ini is placed in the php binary directory.

On the same page the list of php extensions is rather outdated. Some extensions from php3 are still there, some others are missing... I can try to compile a list as of 4.3.0 if you like.

Christoph