I found the following to be useful when configuring WordPress to work with Windows XP, IIS 5.0 and PHP 5.1.6. I assume that IIS already serves up PHP pages and you're getting the "Your PHP installation appears to be missing the MySQL which is required for WordPress" error.
1. Go to http://dev.mysql.com/downloads/ and select "MySQL Connector/PHP"
2. Download both the mysqli and mysql extensions.
3. Move the files that begin with "php_" to your extensions directory (usually c:\php\ext). Overwrite the existing files since these are NEWER and the LATEST from MySQL.
4. Move libmysql.dll to your PHP directory (C:\php). Overwrite the existing file (the one you've downloaded is NEWER). Both ZIP files will have the same libmysql.dll since after PHP 5.0.2 both the mysql and mysqli extensions use the same file.
5. Edit your php.ini file (in C:\php). Ensure the following lines exist and are NOT commented out:
6. Ensure your extensions directory is set as well in the php.ini file:
extension_dir = "C:\php\ext"
7. Configure Windows so it can find PHP, its DLLS, and the php.ini file (this was the CRITICAL step for me, especially the PHPRC variable I describe):
Click Start -> Control Panel
Select the "Advanced" tab
Click the "Environment Variables" button
Under the "System variables" section, click the "Path" variable
Click the "Edit" button
Add "C:\php" to the path (no quotes). Note that each path is separated by a semicolon. Also, you should preferably add this EARLY in the path, since some of the MySQL DLLs may be included in other locations on your computer, and you'd like PHP to find the one installed in C:\php before it finds any other.
Now, click the "New" button (again, in the "System variables" section)
For variable name, supply "PHPRC"
For variable value, supply "C:\php"
Click "OK" repeatedly until you've got all the windows closed.
8. Restart the computer so that these settings will be loaded and recognized by PHP. You can verify that your correct php.ini file is being loaded by testing a simple PHP page with the following:
If your settings are correct, you should see that "Configuration File (php.ini) Path" is "C:\php\php.ini" on the resulting page.