Many times, malicious hackers can gain entry to a computer because they exploit known vulnerabilities in programs that are installed on a wide user base. Take the Adobe Reader program, for instance. It is installed on many computers all over the world, and as such, it has become a target that is used to take control of a computer and install malicious software. Many times, the user will become infected simply by visiting a web page or opening a specially-crafted e-mail attachment designed to exploit the vulnerable software. Often times, the user has no idea that his/her machine has been infected.
Secunia's Online Software Inspector (OSI) will scan the programs on your computer for any known security vulnerabilities. OSI will notify you of the current versions that may be affected and will provide you a link for updating the software. It is a good idea to scan the software on your machine regularly to make sure you have the latest, patched versions of all programs.