go to mydocuments\alpha protocol\apgame\config\apinput.ini and edit the bEnableMouseSmoothing=true to bEnableMouseSmoothing=false and that should fix your problem.
PS. the directory structure may be documents\alpha protocol\apgame\config\apinput.ini instead in Windows Vista and this is all either under your username in users or documents and settings depending on whether or not you are using Windows XP, Vista, or Windows 7. (in order)