1. Once XAMPP is installed:  c:\xampp\htdocs is the home of your content
2. Download Omeka and unzip.
3. Place in the c:\xampp\htdocs directory.  Note: you have the option to rename the Omeka directory.
4. Click on the xampp-control.exe icon in:  C:\xampp
5. Start Apache & MYSQL
6. In your browser enter: http://localhost/
7. Click on phpMyAdmin.
8. The phpMyAdmin control board will appear.
9. Click on Databases.
10. Enter a name for your database (‘omeka’ or use something other than ‘omeka’).
11. In your:  c:\xampp\htdocs\omeka  directory, with your text editor (windows: notepad/notepad++ Mac: text wrangler). Open:  db.ini  file and edit:

[database]
host = “localhost”
username = “root”
password = “”
dbname = “omeka”
prefix = “omeka_”
charset = “utf8”
;port = “”

More details here.