(Halo) I went back

Mouse acceleration in Halo PC described here.

For the link-lazy:

Of Mice and Men
For some unfathomable reason, there is no menu option to turn off Halo’s mouse acceleration, which makes fast aiming all but impossible. Until Gearbox gets Halo’s menus in order, online gamers should create a CFG (configuration) file to turn off mouse acceleration. It’s a very simple thing to do with Windows Notepad. Click on the Start button. Move up to All Programs, then to Accessories. Click on Notepad. In Notepad, type mouse_acceleration 0. Now click on File, and then on Save As. You will want to save this CFG in your Halo game folder. Name it something clever like mouse.cfg. Change the Save as type: box to All Files. Then just click the Save button. Now find your Halo icon in your Start menu. Right-click on it and select Properties. At the end of the Target: box, add a space and the following: -exec mouse.cfg. Click on OK. This tells Windows to run your mouse.cfg as it starts halo.exe.

Thank you VERY much. The mouselag is gone.

In fact, I might restart the game. Gameplay is at least twice as good now :D