My boss got hit with a nasty virus that went through his PCillian antivirus program like it wasn't even there. It was posing as an antivirus program and claimed to have found a virus on the computer and then proceeded to redirect his every attempt to use the internet to their home page. He was then supposed to pay for the full program to remove the "threat"-- when the only virus there is the one they created!
Symantec has a "fix" for this but we tried it and it doesn't work. He finally called Dell and had them help him, and they said this new type of virus- none of the antivirus programs can stop. They had to use some program that one of their engineers wrote when he got hit with this, that can only be used in safe mode. That finally got it out, but it cost him almost a whole day on the phone to recover from this.
If the ultimate defender pops up on your screen, don't hit the "X" to make it go away! It's a fake button that just installs the thing more securely. It's apparently a very sophisticated program and my boss says he never clicked anything on it so if he's right about that, then the thing installed itself with no help from a gullible human. The virus threat has just expanded to new levels of unpleasantness.