1. Did install it using "Run as administrator", if the UAC is ON?
2. Did you remove the 757 entry in the registry after uninstalling it? This one: HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\CaptainSim\B752PRO_FSX for Windows 7 64bit.
3. You could try an offline activation - that might help with the activation.

Is FSX installed in the default location? If it is, you might need to run FSX with administrative rights to get it to work properly.

Mark