I had this before.
I believe it's called vundo or something similar. I don't know of any antiviruses that can completely remove it from your computer - the last 2 times I got it, my computer had to be wiped.
It is called rogue antivirus/antispyware, which means it is tricking you into thinking that you have trojans and viruses on your computer. They want to scam you into paying them to fix the fake error. Ironically enough, it is a virus itself, and other
real viruses or other rogue programs on your computer are a result of this.
An old friend of mine did say that he had a program called
SuperAntiSpyware that did kill it, I haven't tried it myself, but it's worth a shot. I would recommend running scans in safe mode (mashing F9 when starting your computer) so that the viruses are prevented from starting and preventing the fake programs block scans from legit antivirus programs.