You have a spoof virus. It is pretending to be a virus in order to get you to pay a 'ransom'.
This type of malware is often difficult to remove because it knows about real anti-malware (like Norton) and attacks it.
I suggest a trip to you local computer shop to get it fixed. You can avoid this in future by having anti-malware (Like Norton) software always running on your PC.
T, October 2010