1) "Do you know what Unknown Trojan is? DESCRIPTION Unknown Trojan is a new popup error message displayed by a Trojan. It is a simple popup message that may prompt users to click OK to download the rouge anti-spyware program that it advertises. Do not under any circumstances click OK to download this program. Do you get this error message? "System Error! Your computer was infected by unknown Trojan. It’s dangerous for your system (critical files can be lost)! Click OK to download the antispyware program to clean your system! (Recommended)"" Source and further information: http://www.spyware-techie.com/unknown-trojan-removal-guide/ 2) "System error!" "Your computer was infected by unknown trojan. It's dangerous for your system (critical files could be lost)! Click OK to download the antispyware program to clean your system! (Recommended)" I just go this message after I had been going on a Website, searching for some answer to an AB question. Of course, I did not click on the OK button... Neither my antivirus nor my firewall detected this virus. And also not McAfee online scan. I chanced on this trojan removal page and I followed the removal intructions. After this, my system seems to work again without problem. 3) In the google search results in my browser, there was another message: "Your browser was hijacked! Some results was changed by porn advertising! You need to clean your system immediately to prevent it. Download the newest antispyware software!" And there was actually a porn advertising in my google result list.