Problem with sister's computer

Today, my sister was on facebook when her internet came down and her browser was directed to this website or something. It said that she had hundreds of viruses in her computer and the only way to remove them was to download this antivirus thing. She called me over to look at it. The page looked somewhat similiar to the page one would get if one pressed the "my computer" button on one's computer, but a little different. I did not notice this immediately, however, because there were two smaller windows in front of the page. One was asking to download the virus scanner, and the other one seemed to be like the window that one would see if one was downloading a file, but it was at 0%. I immediately pressed cancel on that one, although it did not really go away. I pressed on the page in the background. On the top, it looked like there was a website address, so I knew there was trouble and pressed the red x in the righthand corner, though it did not close immediately. Underneath images of computer drives (they were up at the top of the page), there were red numbers and letters underneath supposedly indicating how many trojans were infecting the computer. One was a 300 number, the other was a 100 number. In the middle, it said something along the lines of the computer being infected by viruses and that one needed to download the software to get rid of them. There appeared to be a scrolldown list of all the viruses, but when I tried to scroll down, it would not let me, almost as if it was an image and not an actual list. The page closed out and I started scanning the computer with Norton antivirus scanner immediately.

Unfortunately, my sister and I had to go somewhere while the computer was scanning. When I came back and turned on the monitor, it looked like the page was back again, although it was blank. I quickly closed out of it, but I recorded some of the address. It went something like (I am not sure if this is completely accurate) www.powerantivirusscanner, then there was a /download/ or /downloads/, then there was something after, which I think was like "Arena-(insert somewhat long number here)" (this is the part which I am very unsure about in terms of what the address was), and then it said .exe.

Things seem to be okay right now, but I want to know what happened. The virus scan did not come up with anything. Is this a virus that is trying to get my sister to download somethng else? Was someone trying to hijack the wireless internet connection of my sister's computer (which my computer does not have- it has a cord)? How can I prevent this from happening, and if my sister is infected, how can I remove a virus when norton does not recognize it?