One simple way to make sure our computer is safe is by applying all security updates provided by software vendors, including the operating system. In Windows, Microsoft MBSA can help us to do that, but that only works for Windows operating system itself and not the other applications that were installed on the system. That's why you need third party such as Secunia PSI.
It's a simple and small application that help users to make sure their computer has the latest security patches and up to date applications by comparing the installed version with the signature retrieved from the Internet. Secunia has a good reputation as a security-related organization, so many people have used the tool.
Another alternative is by using Heimdal Security Agent, developed by CSIS Security Group. It's also another FREE application (for home user) and it constantly keeps a selection of actively exploited programmes up-to-date, while at the same time keeping a close eye on malicious software that exploits vulnerabilities in the monitored programmes.