Windows Security Alert Virus?

Ok, so my brother's computer just got the Windows Security Alert virus. You see, he had Norton anti-virus, but it got deleted when he took his computer to Geek Squad to get it fixed. That was 2 months ago. Last night he said he turned on his computer and a red box came up and said "Windows Security Alert." And for those of you that have Norton Anti-Virus, you know that on your toolbar, there is a little shield, his turned red and had an x in it. I don't know how that happened, because he doesn't have it anymore, but it did. Also, he said pictures of profanity keep popping up. Its freaky. I need to know how to get rid of it and how to prevent it. Please help! Thank you.