Once you've installed XAMPP, download MediaWiki and unzip it into your xampp/htdocs folder. You may need 7-Zip to extract tar.gz files. Rename the mediawiki-xxx folder to wiki.
You'll first need to create a database, which you can do by visiting /phpmyadmin/ on your localhost, typing in the database name and pressing 'Create'.
Now go to /wiki/ and fill out the form. Make sure you select "Use superuser account" since you haven't really created a user for your database.
Click on the "Install Mediawiki" button, and you should have a wiki.
