This is a common issue with laptops. They are designed for intermittent use, and don't take to kindly to prolonged use, especially with HD video.
To help with this situation, put some spacers under the feet of your laptop, buy a cooler, and make sure the vents on your laptop are clean.
You can also select "power saver" in your power options. This will make your pc run slower, but it will consume less power, and as a result, not get so hot!