Slow performance???????????????????

Recently my PC has become dead slow. I've been download quite alot of software and it was on for two days without a rest. It was burning a DVD. I have a laptop. It takes ages for files to loads and I have been experiencing system crashes where applications freeze. I have norton installed and there are no viruses showing up. What has changed because my system was working fine about 4 days ago. It's very sunny here would that effect it because I'm right by the window