Why do i keep getting this annoying pop-up?

here's the pop up that i keep getting:

"system error! your computer was infected by an 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)"

should i trust this pop up and download the program? i ran norton internet security quickscan and windows defender and they don't report any problems

please help

Why do i keep getting this annoying pop-up?adware remover

ur computer is infected by spyware/adwares/malwares, these spyware and adwares r used by web masters to advertise thier website using trojans, dont b worried, u can easily get rid of spywares and adwraes by using any free spyware removal or adware killer tools, u can download them from the google website, below i have wrote a link of google website search that will help u download free spyware removal tool

http://www.google.com/custom?q=download+...

http://darfuns.com/pc-security-tools.htm...

Why do i keep getting this annoying pop-up?antivirus protection

just click on tools..

then pop-up manager..

click blocked pop-up windows
Two questions, what is the name of the program it is telling you to download ? Do the pop ups have messenger service at the top ? Please reply.
Do not click on it, its fake and will make your PC worse. Scan your PC with both of these free.