Free Open Source IDS Intrusion Detection System

Tools and Utilities to Monitor Your Network For Suspicious or Malicious Activity
Snort for Windows
Snort is an open source network intrusion detection system, capable of performing real-time traffic analysis and packet logging on IP networks. It can perform protocol analysis, content searching/matching and can be used to detect a variety of attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, OS fingerprinting attempts, and much more.
Sax2
Ax3soft Sax2 is a professional intrusion detection and prevention system (IDS) used to detect intrusion and attacks, analyze and manage your network which excels at real-time packet capture, 24/7 network monitor, advanced protocol analysis and automatic expert detection.
Bro
Bro is an open-source, Unix-based Network Intrusion Detection System (NIDS) that passively monitors network traffic and looks for suspicious activity. Bro detects intrusions by first parsing network traffic to extract its application-level semantics and then executing event-oriented analyzers that compare the activity with patterns deemed troublesome. Its analysis includes detection of specific attacks (including those defined by signatures, but also those defined in terms of events) and unusual activities (e.g., certain hosts connecting to certain services, or patterns of failed connection attempts).
Prelude
Prelude is an "agent less", universal, security information management (SIM) system, released under the terms of the GNU General Public License. Prelude collects, normalizes, sorts, aggregates, correlates and reports all security-related events independently of the product brand or license giving rise to such events by normalizing them to a single format called the "Intrusion Detection Message Exchange Format"
OSSEC
OSSEC is an Open Source Host-based Intrusion Detection System. It performs log analysis, file integrity checking, policy monitoring, rootkit detection, real-time alerting and active response.

you can find it very easy anywhwer

you can use a program called snort