Yes, that's Windows' own acceleration so keep it disabled. Also keep the slide in the middle (6th setting). Most games these days either use raw input by default or have an option to turn it on. This bypasses the OS tampering with the mouse movement signals and allows the game to pull the raw mouse data, bypassing any tampering with the Windows pointer speed slider.
The reason the slider is bad on anything but 6 is because it can cause inconsistent mouse movement. With it below 6, it simply throws away mouse counts, reducing the effective DPI. It throws away every 4th count, every 3rd count, or every other count, etc. until it's only sending 1 count for every 16 that the mouse sends. Putting the slider above 6 has the opposite effect. For every 4 counts the mouse sends, the OS will move it one more. Go a bit higher, and it's every 2 counts the OS sends one more, then it sends an extra count for every one received by the mouse. At the highest setting, it actually sends 7 extra counts for every 2 that the mouse sends, effectively making the mouse move 3.5x as fast. You can see the problem here tampering with the counts if you want pixel perfect accuracy. Lowering it means that the mouse might not actually move when you want it to or think it should, and increasing it will make the mouse start skipping pixels and might make it hard to hit a certain target or click a small button.