Trojan.Zlob-X.a

Hi, i really have no idea what im doing when it comes to this stuff, and since yesterday a popup keeps appearing everytime i open a new page in IE, (it reads: your system is probably infected with the latest version of Trojan.Zlob-X.a Full system optimization will greatly increase your computer's performance and prevent data loss. Click okay to download anitspyware software!) and when i run a check my anitvirus software finds nothing. Im quite confused. Please help.

thanks.