Some antivirus software may prevent the client from starting, in particular by limiting the amount of memory that the program can allocate. This amount of memory might not be sufficient for the player even to create its network buffers.

One of the antivirus programs showing this behavior is BitDefender. It's possible to find traces of the same problem here:

https://www.nomachine.com/forums/topic/possible-permission-problem-on-windows-x64

And here:

See here: http://forums.thesecretworld.com/showthread.php?t=48919&page=35 for the same problem

and here: http://forums.thesecretworld.com/showthread.php?t=77016 for the same problem affecting some games.