Live for Speed allows when you first install the game. This is intended as a test. You can reset this by reinstalling Windows (overkill) or using a virtual machine, but that is not practical.
DLL injection and registry hacks can corrupt your Windows installation. Many users report that after using an LFS unlocker, other games (like Assetto Corsa or rFactor 2) start crashing due to modified system libraries. lfs s3 unlocker 7d
The "S3" refers to the specific security algorithm generation—the third iteration of the Siemens/VDO lock system. The "7D" is the firmware/hardware revision. Early unlockers were slow, brute-force devices. They might take 45 minutes to unlock a single module, and if the car battery voltage dipped, you bricked the unit. Live for Speed allows when you first install the game