| Make sure your game files are correct by using the game file validation program in Steam -> My Games -> Properties -> verify Game Cache (I think that's how you get to it - Correct me if I'm wrong)
Also like others have said, update your video drivers, and set the affinity on your processor IF you have a dual core. Task Manager -> Processes -> Right click hl2.exe -> Set Affinity -> Pick one (I'd choose core 0 just cuz it's zero)
Hopefully that helps. If not, reinstall source I guess.
In response to Shed:
64 Bit code runs much faster. All 64 bit coded processes, such as new graphics(x64 - meaning windows 64 bit) will run faster than a 32 bit driver on a 32 bit CPU.
If you've got a 64 bit CPU and are running 64 bit apps, you will notice an increase in speed. However, if you are running 32 bit apps on a 64 bit CPU, I think you won't increase in performance, or will see very slight gain.
Now seeing how your drivers are windows tasks and processes are in 64 bit code, that means more CPU is free for your games to play, if you use an AMD which can run both 64 and 32 bit code naturally(without emulation - which makes latency).
Lamens Terms: If you play CSS/BF2/2142 on a 64 bit AMD system while using a 64 bit OS, you will see an performance increase such as I did. Several frames on a 1.6ghz system, single core.
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