Right-click the trainer .exe , go to Properties > Compatibility , and set it to run as Administrator and in Windows XP (Service Pack 2 or 3) mode.
If you want even more control beyond a standard trainer, many players recommend the mod. This tool allows you to unlock all cars (including hidden police and traffic vehicles) and customize game settings via an .ini file.
Mastering Rockport City: The Ultimate NFS Most Wanted Trainer 1.3 Guide