you find out who is responsible for the site (registrar, hosting company) - do this using a whois database - and email all of them about the issue, and which software it tries to install.
ask them for updates, so you know how serious they take your email and how quickly they take a look at the page. don't email the owner, as this can give him/her time to change parts of the site. in this way i have drawn attention to a few online banking frauds, and in some cases the pages were taken offline by the hosts withing 2 hours.
