Laptop problems (sudden crashes)?

I have an Advent Windows 7 laptop which I got last Christmas, anyway I have scanned (full) my C: drive with my Anti-Virus AVG and Malware Anti MalwareBytes. But my laptop crashes suddenly and the blue screen of death appears randomly. I honestly don't know what to do or what I can do to fix it. I'm not willing to bring it to a Repair shop because it would cost a lot for reparations.