Large companies should implement an IDS (Intrusion Detection System) - a line of defence that detects hostile activity on a network. While such systems are expensive, and sometimes hard to use, you can create an ad hoc IDS by combining a firewall, anti-virus software and vulnerability assessment utilities. It's possible to scan for potential security gaps using two processes that are commonly employed by hackers themselves. Port scanning checks against the some 65,000-plus ports a PC can use to communicate across networks. Packet sniffing software analyses data as it travels across networks and is used legitimately by network administrators to monitor network traffic and identify bottlenecks. Unfortunately, unencrypted usernames and passwords are also often transmitted across networks; hackers can use packet sniffing software to detect such important data.

Most security scanner software is aimed at large companies with price tags to match, but there are a few free applications including the Microsoft Baseline Security Analyser 1.2.1, although it is quite difficult to use. You can download the security scanner (currently at vers. 2.00) here - http://www.microsoft.com/technet/security/tools/mbsahome.mspx