Google has found that almost 1 in 10 websites has some sort of malicious code..
http://www2.csoonline.com/blog_view.html?CID=32927
[Editor's Note (Valle) Google has confirmed what many security companies having been saying for a while: Many websites are now compromised and making bot nets out of their victims. This study also validated new malware trends:
1) Hackers are using scripting languages to determine how a computer is vulnerable and then requesting an appropriate exploit from a centeral server.
2) Hackers are changing binaries to get by AV programs.
As per the previous post, the IronPort C series appliance would help enterprises stop trojans and malware from being downloaded and as another layer of defense CSA could be recommended as well