install the latest drivers for your graphics card, and go to Microsoft's website and install DirectX 9.0c
also, Leonidas was asking what graphics card you had. your graphics card may not be very fast causing HL2 to be slow. HL2 and Doom 3 require a lot more graphics card power than previous games
if you want to upgrade your graphics card, try and get at least a Radeon 9600 XT, or if you're willing to spend more money, get a Geforce 6800