i think it's so hard to going inside a secure network ...
a secure network is the network where the users are little smart and have common sense "where they will click and where they will not" .. just it.
just make secure your network with basic security software .. like antivirus and firewall ... but the most important thing is; your user must be smart for using internet and will not click any unknown link and will not use any toolbar .. even yahoo toolbar!!!
it's my thought ...