Its probably because you're using the Software mode for your 3D engine. Does it crash when you make a Spirit Bomb or swoop at someone?
From the main menu of Half-Life or ESF, click on Configuration --> Video --> Video Modes.
Select OpenGL. (If your card doesn't support OpenGL, you have to use Direct3D. It makes more lag, but it works) And select a screen resolution that best suits the power of your PC.