Something on my computer

Over the summer my laptop began to act strange. It was starting up just fine but only worked for about 5 minutes then just slowed way down to a snail's pace. I couldn't figure out what was wrong and Dell wasn't much help either. Then I came across a section in my manual that talked about restoring my computer to its original factory specs. This basically meant that when I performed this procedure, my computer would be the way it was when I got it from Dell. I was amazed that the initial setup was tucked away deep in my computer files so I backed up all my important documents on an external hard drive and performed the system dump. 10 minutes later my computer worked like a champ and hasn't had a problem since. When I was able to ask around and see why this was the case, I was told that maybe a virus was stuck in one of the files that got erased off the hard drive completely. Whatever it was was irrevelant. My laptop was back up and running.