From what you are saying here, it seems to me that your Antivirus program did'n quite did it's job. It didn't protect your PC properly. It happens so often. It happened to me too and to solve the problem I changed the antivirus I had and switched to Kaspersky. It is doing quite a good job, I didn't had any problem with my Pc since I installed Kaspersky. If you want too, you can find it here: google.com