Ms ISA Server's (proxy) Filter against trojans and backdoors

Dear All,

I'm a lan administrator in my firm. We've got a proxy which run under an windows 2000 Server with a Microsoft ISA Server. I would like to make a Filter against trojans and backdoors. Can you should me any softwares (free or not, but free better).

I've found a little solution which is update the blacklist's internet site but i don't know where to find a .txt blacklist as the SpyWareBlaster's.

Thanks,

Hugo