I've learnt a lot in a week...
I recommend, lamadame, that you read through the appropriate hack documentation as found on wordpress.org. You have indeed been hacked, and you'll find all new users you add will take on this name (sec-w.com), you may also find a some of your functionality in the backend has vanished. Though I also could see no change on the front end of my site, I wasn't taking any chances and stripped it back to nil and started over.
I have much more secure and hardened sites now and there are a few things I was doing which I won't do again.
Don't use 'admin' as your username, this opens you up to brute-force attacks. Also password strength is key for this, in fact a lot of hacks seem to happen this way. So firstly I would go ahead and change just about every password imaginable.
Do make sure you're themes, plugins and WP itself are all up-to-date. Don't leave deactivated plugins or themes just lying around - remove them, they may pose backdoor threats.
I've altered .htaccess files within root dir and wp-admin dir to deny many bad IPs. and; in my case I've allowed only my own IP access to wp-admin (may not be suitable for you).
I've also installed the following plugins and am assessing their usefulness and stability as time goes on. These all came via strong recommendation, although I am suspicious on using so many security plugins:
• BackWPup (I receive an email backup, daily, of vital files)
• Exploit Scanner (this one will comb over your site and assess risks)
• OSE Firewall (has blocked some interesting attempts, I receive notification of anything untoward)
• Simple Login Lockdown (great for bruteforce attacks)
• WordPress File Monitor Plus (emails every time a file is changed, whether it's me, automated, or nasty)
I also recommend changing your database prefix from the default 'wp_' to something, anything, unique.
All I can say is, backup everything you have now and get ready to READ READ READ... I feel much more confident now and may have omitted some of my steps from this post, the only way is to grow your understanding.
Hope this helps. Good luck, mate.