EventSentry Light is a powerful tool that can monitor your Windows event logs (and system health), alerting you to any problems.
If you're a Windows expert and want to be warned if a particular event type occurs, for instance, then you can have the program display a message or send you an email.
A Software Monitoring option allows the program to log when software is installed, or uninstalled (useful if your PC has several users and you'd like to track what they're doing). And it can also record programs that have configured themselves to load when Windows launches.
A Disk Space monitor is able to alert you when the free space on a drive gets too low (you get to decide what figure that might be). Or it can check a particular folder, alerting you when its size exceeds or falls below a particular limit.
And the Performance Monitoring module can track your PCs CPU or memory usage, again logging the details if something's hogging all your resources.
There's plenty to like about this free version of EventSentry, then, but it also has many restrictions that are removed in the commercial version. It can't monitor as many logs and type of logs, it's far less able to monitor log files, and many features are available only in a limited form: EventSentry Light can only monitor the size of one folder, for instance, while the full version will track as many folders as you like. So if you need more power, then consider upgrading - prices start at $85, and the EventSentry site will tell you more.