I installed Apache 2.2, MySQL 5, PHP 5, and phpmyadmin on winXP recently, but when I try to access http://localhost/phpmyadmin/index.php I get an error that says "cannot load mysqli" extension. I followed the instructions here: http://www.devside.net/guides/windows/phpmyadmin

Under the php configuration settings, it tells me to "Edit file %SYSTEMROOT%\php.ini". Is System root just a placeholder for where the path to php.ini is, in my case C:\Server\PHP?

The directions tell me to uncomment ";extension=php_mbstring.dll", but I don't have that line. Instead I have "extension=msql.dll", so I commented this out and replaced it with ";extension=php_mbstring.dll". Unfortunately, that didn't work either. Any ideas what could be wrong? Thank you.

You should edit config.inc.php and change value of the line to mysql instead of mysqli.
The other extensions you've mentioned have nothing to do with mysql.
As for %SYSTEMROOT%, I guess it's c:\windows\system or c:\windows\system32

Open the command window and enter "echo %SYSTEMROOT%". On my PC is says "C:\WINDOWS". That's where your php.ini wants to be, I think. Just copy it from C:\Server\PHP to C:\WINDOWS (or whatever your SYSTEMROOT shows).
I use XAMPP (http://www.apachefriends.org/en/xampp.html) which is self-contained and keeps all of the configuration files within it's directory.

Thanks for the help, nothing is working so I'm just going to uninstall everything and try XAMPP