Hi segoviafoto, welcome to the forum
Sorry, I don't speak spanish, so someone will have to translate for me
Unfortunately it appears that the site is infact infected.
There is a script that points to a site that is blocked by avast. This script has been seen many times recently on the forum, and appears to be related to theming.
One method of attempting to solve it is to use Exploit Scanner by Wordpress:
This is from a thread which had the same type of script embedded in the pages.
(
http://forum.avast.com/index.php?topic=92742.0)
Polonus had a link to a scanner that helped identify the issue, it may be of some help to you:
...
I found the problem. It was old timthumb.php file!!!
Exploit Scanner show me all of infected files and now everything is ok with my website...
Well the sucuri alert is not for an outdated WordPress version, the alerts is foir that specific theme: wordpress London live theme
Use the Wordpress exploit scanner: http://wordpress.org/extend/plugins/exploit-scanner/
This plugin is far from perfect, so you might have to plough through the code for changes yourself,
You fell victim to a php hack so you have to secure the use of that first,
polonus
Scott