The following instructions tell you how to installed Ioncube on xammp

1. Find your PHP version. You will need to know the version when you download the ionCube loaders.
2. Download ionCube loader from: http://www.ionCube.com/loaders.php
   
   Download this version: Windows (x86) -- Windows Installer  (for windows etc..)
    
3. Create a folder in ../xammp/apache/ioncube (can be any folder of your choice)
4. Extract contents of the Ioncube Loader archive in to the folder above
5. Open ../xammp/php/php.ini
6. Add the following line before any other zend_extension statements.
   
   zend_extension_ts = "D:\xampp\apache\ioncube\ioncube_loader_win_5.2.dll"
    
7. Stop and restart xammp

If successful you should now see a section in your PHP Info page that says: (just do a  page search for ioncube)

Additional Modules
Module Name ionCube Loader

These slightly revised instructions allow you to run ioncude successfully on xampp 1.7.3 and on Windows 7 64-bit

I found the solution...

This should be added to the php.ini regardless of the thread safety status:

    zend_extension = "C:\xampp\ioncube\ioncube_loader_win_5.3.dll"

This is wrong but it is in the documentation:

    zend_extension_ts = "C:\xampp\ioncube\ioncube_loader_win_5.3.dll"

It seems like zend_extension_ts does not work anymore.