Snort is a NIDS, which is offered as a free open source software product. The program itself is free of charge, but you'll need to pay if you want to have a complete, up-to-date set of threat detection rules. Snort started out as a one-man project, but it's now owned by Cisco.  Understand though, this isn't something that you install on the machine that you want to protect. Rather, you'll have at least one dedicated Snort machine someplace on the network, just monitoring all network traffic, watching for anomalies. When it sees traffic that shouldn't be there—something that indicates the presence of a bot, for example—it can either just send an alert message to an administrator or it can even block the anomalous traffic, depending on how the rules are configured. For a small network, you can have just one Snort machine that acts as both a control console and a sensor. For large networks, you could have one Snort machine set up as a control console and...