You can open the site in Windows Notepad to see the code and to prevent it from executing in a browser window.

DO NOT OPEN THIS LINK IN A BROWSER! USE NOTEPAD
Here's how:

1. Click "Start" then "All Programs"
2. Click "Accessories" then "Notepad"
3. Click "File/Open"
4. In the "File name" box enter "http://www.aryan"
5. Now enter ".es/img/ken.gif" (after "aryan" and without quotes)
6. Under "Files of Type" select "All Files"
7. Click "Okay"

This explains how the user's machine gets infected but not how the hacker gained access to your server.

Check your permissions and ask for help from your host. They may be able to help you in tracking down the problem.

If you have one, I'd check your .htaccess file to a mod rewrite that points the user to the malicious code. Check your server for "response.redirect" code as well.