Secuity Tool is a rogue (fake) antivirus program created in 2009 that reports false viruses to your computer to get you to install it. When you install it, it actually infects your computer. It is a member of the Win32/Winwebsec family of rogues. It's name is similar to that of System Tool, another virus in it's family.
Like many other rogue antivirus programs, when installed, Security Tool scares the user by constantly saying that the computer is infected; by saying things like: "WARNING! YOUR COMPUTER IS INFECTED! 55 INFECTIONS FOUND!" When this happens, Security Tool blocks you from using your internet browser. The number of infections is randomly generated, so it never matters if it scares you. Then it will say something along the lines of: "Some of your software may have been edited by this malicicous software, this may cause software loss or instability. Click here to remove this with the full version of Security Tool (Recomended)." Another symptom is that your screen will go black, lose all icons, etc.