It would help if the original poster would let us know what Operating Systems he/she is running on the network so we could make recommendations for him/her...
If on *nix, start with nmap, look at *ALL* your open ports, and decide if they *NEED* to be open, closing those that dont.
(Also KILL OFF TELNET <G>... install SSH <G>)
Then as another responder mentioned, give saint a spin ... another that I like is nessus (
Then just for the ultra paranoid (such as most people should be...) grab snort, learn it, use it, monitor it, love it...
For Win32... best way to secure yourself is to either
a) Unplug the network cards
b) install */BSD or Linux
<just kidding... maybe...>
I dont use/support/recommend any microsoft product, as they are reliable on in that they are reliably late, insecure, unstable, and overpriced, so follow the advise of others here who use the M$ stuff..
-John