Pop Up Notification

When I was on facebook I was trying to go to a different page on facebook and instead of going to that page it looked like my internet browser closed and then this box came up that was saying that I had viruses in my shared documents, my documents, and the hardrive. It automatically did this scan thing, but I'm not completely sure what it was, but it tried to get me to download this program. I didn't download it because my computer's spyware protection wouldn't let me. I called one of my friends who is pretty good with computers and he told me to download the Adware program, and so I did. Then I ran the scan and it told me that I had 513 problems, but without paying for something it wouldn't fix it. I just wanted to know how I could make sure whatever those problems were weren't going to affect my computer.
Thanks.