My computer won't turn on. Please help?

Hi everyone. So about a month ago, I was on my computer and a random popup appeared saying something needed an update. I didn't know what it was so I kept clicking no. It kept re-appearing (like 20 times). Then, I turned it off. I tried to turn it back on but it went to this startup repair thing and checked for problems. It couldn't fix the problems. I tried turning it off, taking the battery out and putting it back in. But the same thing keeps happening. Basically when it says the repair system can't fix the problem it gives me the options of finish or cancel. Cancel restarts the computer and it goes back through the whole startup repair process where it fails. Finish just shuts the computer down. I know its some sort of virus, but I don't know how to fix it. I made the mistake last time of not saying what operating system I'm using so I'd like to add that my computer is a Windows 7 Sony VAIO. I'm don't really understand a lot about computers and their operations (my sister had to help explain some of it to me, but she still doesn't know much) so if you could try to speak as close to layman's terms as possible that'd be great. Thanks.