Secure systems on a budget

As the title suggests, does anyone know of any free or inexpensive software that will enable me to monitor the actual usage each LAN member is using, & what on. I have a 10MB LAN extension service between our 2 offices that is constantly choked but I cannot understand why?
Cheers
G

Have a look at Nessus. The server runs on Linux/BSD/Unix only (to my last recollection) however, a number of excellent Windows clients exist. Both Server and Client are free.
http://www.nessus.org

The Knoppix Live CD (boots from CD and runs in RAM) is a fully self contained GUI based OS (Linux) jam packed with useful programs. It has nmap, Nessus (server and client - very easy to setup and run), as well as a number of other testing and auditing tools (ethereal, etc.).

The best part is you get the functionality and programs of Linux without needing to install it anywhere.

Among other things, it has some very good wireless tools on it (airsnort, kismet, etc.), and you can also use it to look at hard disk contents without running the native OS, so it can also work as a forensics tool.

And next time a worm has gotten you down, you can check your mail with it too.

www.knopper.net/knoppix/index-en.html

Microsoft MBSA (baseline security analyser)
Microsoft SUS System Update service
.. not ideal but it helps reveal and solve the bigger / obvious holes.
HP have a variety of tools for tehir HP-UX .. some free (patch management / system audits / intrusion detection)
Use chkrootkit on any Linux boxes ..
Use LIDS on Linux boxes
Use md5sum checking on any files you care about

.. for a few hundred pounds there are several network security check / fix tools depending on the size of your network (some are charged per client).

In summary .. pay for tools or spend lots of time and effort managing your patching / security checking / intrusion detection.