This is a tiny ASI plugin (injectable DLL) that prevents games from accidentally enabling mouse acceleration on Windows.
Place a ASI/DLL loader like ThirteenAG's Ultimate ASI Loader
in the game's folder and place the VariableMouseAccelerationFix.asi in either
the same folder or a scripts subfolder.
You can check if it's working by checking if "Enhance mouse precision" does not get enabled when you run the game in question.
Another way to check if it's working is by using the "MouseMovementRecorder"
tool included with MarkC's Mouse Acceleration Fix.
If the EnPtPr column doesn't become "On" with a red background, you're good to
go. Other columns may still become red due to old games re-centering the mouse
cursor to calculate movement between frames, but this doesn't mean acceleration
is on if EnPtPr remains "Off".
Old Windows games (usually pre-Windows 2000) may call a function named
SystemInformationInfo to temporarily disable mouse acceleration, so that they
can calculate mouse movement in a linear manner.
Unfortunately with Windows 2000 onwards Microsoft changed the way mouse acceleration works and introduced a strange quirk in which calling the function mentioned above enables mouse acceleration (known as "Enhance pointer precision" in Windows' mouse settings).
This causes older games to behave with accelerated aim, hindering accuracy and precision with quick movements.
Another quirk with this is that if the game crashes before it can restore the mouse acceleration settings back to normal, the OS will remain with it enabled until the user manually turns it back off.
A famous fix for this is the aforementioned MarkC's Mouse Acceleration Fix, which works around this quirk by altering the acceleration curve in the Windows registry to be linear, thus making so that mouse acceleration being enabled or not doesn't matter: movement will always be linear.
That's a good approach, but it relies on working around the issue with a hack and naturally requires different configurations for different DPI levels and operating systems. There's also no guarantee it won't be patched back by a Windows update and must be re-applied on every OS install.
It's still advantageous in the way that it'll work on every game without further configuration or installing plugins though.
This plugin takes a different approach: it hooks into the function and waits for it to receive parameters to alter mouse settings, and when it does it simply does nothing, ignoring the request while telling the program the request worked as normal.
Because of this, it entirely bypasses the quirk of enabling acceleration, it will never be enabled to begin with, by simply loading the plugin with a loader like ThirteenAG's Ultimate ASI Loader.
If the game offers an option to use raw input, you should use that and don't need bother with either this plugin nor the previously mentioned fix.
Raw input simply bypasses Windows' mouse acceleration entirely and does not modify the user's settings in any way.
DirectInput basically does the same for mouse input, so if a game uses that you also don't need a fix.
Games have come a long way since Windows 9X, so these workarounds are only really needed with older games that use antiquated methods to gather mouse movement, newer games that use modern approaches to handle mouse input won't see any differences with these fixes.