1. Open Steam Library, right click on CS 1.6 and choose Properties
   
   
   
   
   
   
2. In General tab press Set Launch Options button. Write down (or add in case there is some text already) -noforcemaccel. Press OK.
   
   
   
   
   
   
3. In Windows open Control Panel, find Mouse Properties, open it, go to the Pointer Options tab and uncheck Enhance pointer precision. Press OK.

If you feel that your acceleration suddenly breaks down from time to time, use RInput programm (you can find more info here).

Properly speaking, one need to mention m_rawinput cvar: value 1 means that acceleration will be determined by application (that is CS), and 0 means that mouse options will be defined by Windows. There are substantial grounds for believing that this cvar works incorrectly, but you should set it to 1 (it's impossible to turn off acceleration for some mouse models when m_rawinput is 0).

Finally, a few words about m_filter cvar (you can also change this cvar in CS Options in Mouse tab - it's called Mouse Filter). Actually, it's not connected with acceleration, this cvar makes your mouse movement smoother (when set to 1). This effect is caused by flattering the dependence of angle speed of mouse movement on time: angle speed at each time point is calculated as a half-sum of two latest values.