One of my users is experiencing this pop-up message constantly while using her computer. I have noticed it as well. She is running Windows Vista Home Basic. She has antivirus installed and updated. I ran AVG antivirus, antispyware, and windows defender all multiple times. I removed whatever entries were found, but nothing seemed relevant to this error that the user is getting: "Critical System Warning ! - Your system is probably infected with latest version of Trojan. Zlob-X.a full system optimization will generally increase your computer's performance and prevent data loss."