System Tool is a rogue (fake) antivirus program. It is a member of the Win32/Winwebsec family of rogues. It shares it's logo with MS Removal Tool and Essential Cleaner, two other rogues in it's family.
BehaviorWhen run, System Tool will hijack your desktop background, and replace it with a message that says "YOUR'RE [sic] IN DANGER! YOUR COMPUTER IS INFECTED WITH SPYWARE!" and automatically run a scan. System Tool will then proceed to attempt to scare the user into updating his/her version of System Tool by reportng fake threats that can only be terminated by updating, but whenever you try to do that, it will tell you to activate it.