If you want to get the best possible performance from your game please
disregard the posts such as "PREFECT SETTINGS" because they apply to very specific cases.
If you experience Lag or Crashes It may be that you are using the cs_militia map. If this is the case, go into your in-game options and disable HDR. If the problem continues simply do not use the map.
If you still experience Lag try lowering your effects settings in the in-game options. things like lowering the Material Resolution, and other fancy things may increase performance.
If you are still having problems after lowering the settings to LOW try changing the resolution of the window. maybe something less than 1024x720. I know it's nice to ahve the bigger window but if your computer can't handle it don't use it.
If you still have issues change your launch settings accessible via STEAM and add something like either
-dxlevel 70
-dxlevel 80
-dxlevel 81
This will lower the Direct X standards crossover will try to use when playing thus maybe fixing errors
If you have your resolution on super low, your effects and graphics minimalized, your "-dxlevel" set to 70, and your using a really lowres map AND your STILL having lag issues then either:
Your CS:S is corrupted and must be reinstalled, Your bottle is bad and must be reinstalled, you pirated a bad version of the game and must by face-palmed, or YOU COMPUTER IS TOO OLD!
If you computer is too old (The MINIMUM official system requirements are thus :
Minimum: 1.2 GHz Processor, 256MB RAM, DirectX 7 level graphics card, Windows 2000/XP, Mouse, Keyboard, Internet Connection
Then you need a new computer.
Hope this helps