Heres a usefull resource for internet seurity. I'll just quote the first part of the document and for the rest you can follow the link

The SANS Top 20 Internet Security Vulnerabilities

The vast majority of worms and other successful cyber attacks are made possible by vulnerabilities in a small number of common operating system services. Attackers are opportunistic. They take the easiest and most convenient route and exploit the best-known flaws with the most effective and widely available attack tools. They count on organizations not fixing the problems, and they often attack indiscriminately, scanning the Internet for any vulnerable systems. The easy and destructive spread of worms, such as Blaster, Slammer, and Code Red, can be traced directly to exploitation of unpatched vulnerabilities.

Four years ago, the SANS Institute and the National Infrastructure Protection Center (NIPC) at the FBI released a document summarizing the Ten Most Critical Internet Security Vulnerabilities. Thousands of organizations used that list, and the expanded Top-20 lists that followed one, two, and three years later, to prioritize their efforts so they could close the most dangerous holes first. The vulnerable services that led to worms like Blaster, Slammer, and Code Red, as well as NIMDA worms - are on that list.

This SANS Top-20 2004 is actually two Top Ten lists: the ten most commonly exploited vulnerable services in Windows and the ten most commonly exploited elements in UNIX and Linux environments. Although there are thousands of security incidents each year affecting these operating systems, the overwhelming majority of successful attacks target one or more of these twenty vulnerable services.
source - SANS

Enjoy but be warned some of it makes for farily heavy reading.