The following instructions are for installing apache and php on Windows XP:

1. Download the apache msi from the apache website
2. Run the msi, you can use localhost for the Domain and Server.  The admin email can be what you like
3. If you choose the default install dir, the files will be placed at C:\Program Files\Apache Software Foundation\Apache2.2
4. By defualt the "home" directory is htdocs
5. If you are running IIS on the same server, it will be using port 80, so you need to change apache to use another port to do this edit the conf\httpd.conf file:
   
   Listen 8000
   
   ServerName localhost:8000
   
   
6. Restart apache, then you should be able to browse to http://localhost:8000
7. If you want to change the home directory edit httpd.conf:
   
   DocumentRoot "C:\All\htdocs"
   
   and change the <Directory...> to
   <Directory "C:/All/htdocs">
   
   
8. Next for the php install, download the latest php zip file (I downloaded php-5.2.8)
   
9. Unzip to program files\php-5.2.8
10. Copy php.ini.dist to C:\Windows and rename to php.ini
11. Now we configure apache to run php
12. Copy php5ts.dll to the bin dir under your apache install directory
    
13. Edit http.conf to include:
    
    LoadModule php5_module "C:/Program Files/php-5.2.8/php5apache2_2.dll"
    AddType application/x-httpd-php .php
    AddType application/x-httpd-php-source .phps
    
    
14. Restart apache
15. Create a file in your home dir (by default htpdocs) called info.php and add:
    
    <? phpinfo(); ?>
    
16. Browse to http://localhost:8000/info.php and you should see your system info
    
    

I was recently setting up WordPress to run locally on my development machine and when I tried to browse to the WordPress homepage locally, I recieved the error:

Your PHP installation appears to be missing the MySQL extension which is required by WordPress.

My configuration:
Windowx XP
Apache 2.2
php-5.2.8
MySQL 5.1

To correct this issue, I did the following:

1. In the Php.ini file (on my machine this is located at C:\Windows):
   Set extension_dir = "C:/Program Files/php-5.2.8/ext" (where php is installed on my machine)
   
2. uncommented (remove ;)
   extension=php_mysql.dll
   extension=php_mysqli.dll
   
3. Copied libmysql.dll from php dir to system32 (C:\WINDOWS\system32 on my machine)

Hope that helps anyone with a similar issue.