To disable it you must use the noforce commands.
1. Open control panel and open mouse properties.
2. Uncheck 'Enhance pointer precision' option in pointers tab. Dont worry, it wont degrade any precision of pointer or something, this option is meant only for low DPI mice back some years.
3. Click apply and ok.
4. Open steam, goto games list, right click on the Counter Strike 1.6 and select properties.
5. Click on set launch options button and in the text field type this
-noforcemparms -noforcemaccel
6. Click on ok, and run the game.
7. For non-steam users create a shortcut of hl.exe and in its properties put -noforcemparms -noforcemaccel just after 1 space (after double quotes) in the target field.
Tip about mouse acceleration:
Mouse acceleration should not be turned on while gaming, its not good in gaming, as you can miss the targets easily, specially in First-Person-Shooter games like Counter-Strike 1.6, Quake 3 Arena etc. It can be turned off by unchecking the "Enhance Pointer Precision" in control panel, but in some games its on by default, like Counter Strike 1.6.