Hi!
I have good news for you: With your software (Apache, PHP, MySQL) you do not have to do any additional updates. Updating only MediaWiki is enough.
Manual:Upgrading is mainly targeted at users with a Linux server; but you have Windows XP. So you have to adapt parts of the commands, which are presented there. E.g. you cannot just type "php update.php" on the Windows command prompt to run the script update.php with php.exe. Instead you would first have to change directory to the path in which you have update.php and call php.exe by typing the complete path to the exe.
In short to do the update you must do two things:
First replace the old MediaWiki files with the new ones and second update your database.
That means: Download the tar archive with the newest source code (version 1.19.2) and extract it on your PC. Check which files and folders you have there; these are the ones, which you have to replace in your MediaWiki installation. Move the old files and folders out of the way (maybe best do not delete them, but only move them to the dustbin, just in case something goes wrong) and put the new files and folders there.
Make sure your Apache server and MySQL are running. Now update the database. Two options: The easy one is to use your webbrowser. Visit the URL, which basically points to your wiki, but do not let it point to the MediaWiki mainscript index.php, but to mw-config/index.php. This is a web updater, which does the needed database updates, when you go it through. Second option would be to do the database updates by running the file maintenance/update.php on the Windows Command Prompt with PHP.
To run update.php via command prompt, if you have MediaWiki in C:\Mowes\MediaWiki, you would first cd to C:\Mowes\MediaWiki\maintenance. If you have php.exe in C:\Mowes\php\php.exe, you would then execute php.exe with C:\Mowes\php\php.exe update.php like so:
Microsoft Windows XP
Copyright Microsoft Corp. 1985 - 2001
C:\Documents\Terminator> cd C:\Mowes\MediaWiki\maintenance
C:\Mowes\MediaWiki\maintenance> C:\Mowes\php\php.exe update.php