Laptop is slow and freezes?

It's been acting up for a while now. Seems to be getting worse. It keeps freezing ad well. I was on YouTube then it froze and although the fan was still going the picture was up and frozen, the mouse wouldn't move and task manager wouldn't come up so I had to hold the off button. This is so annoying. My dad said it's slow because if broadband but why is it freezing?