I have an Acer Aspire 5520G and recently I had a big problem with it. A couple of days ago I was watching a video on the internet and my laptop froze. I had to shut it down after waiting a couple of minutes and when I wanted to restart it, it acted normally for 2 seconds then it restarts automatically over and over again. I can only shut it down by unplugging it. Nothing appears on screen. I think that it is a hardware problem. If you need further information please leave comment.
Apparently this a a common problem with this particular model of laptop. If it is still under warranty, (unlikely given its age) contact Acer for assistance.
Apparently some people have had success correcting the problem using a heat gun. That might sound weird, but it's just a roundabout way of reflowing the northbridge, which was poorly soldered to the motherboard and causes this problem. Unfortunately many people report this "solution" to be temporary, and I wouldn't recommend messing with it.
Personally, I would just buy a new laptop. You could replace the motherboard, but it will probably be cheaper and easier to get a new one instead.