Go to C:\Users\<YourUserName>\AppData\Roaming\CaptainSim\x572, or open the Run Command (Windows Key + R) and enter %APPDATA%\CaptainSim\x572, click OK, then delete the key_x572.dat file.
This will allow you to reinstall the 757 Captain III (757-2PW) Base Pack 1.7 [FSX/SE/P3D3], needing to reactivate it using Instant Online Activation, or Manual Offline Activation.


Note. I suggest you delete all the relevant files for the 757 III in FSX:
FSX\Captain_Sim\757-III
FSX\SimObjects\Airplanes\CS_B757-200-III_PW2037
FSX\SimObjects\Airplanes\CS_B757-200-III_PW2037_4D
FSX\SimObjects\Airplanes\CS_B757-200-III_PW2040
FSX\SimObjects\Airplanes\CS_B757-200-III_PW2040_4D
making it a totally new/clean install.