Did you also completely disable the user account control (UAC)? That problem usually occurs when the setup.exe is unable to write into the Windows Registry. I noticed that, in case you installed the client to C:\Program Files and the user account control is enabled, I also get the same problem, but if I disable the UAC and install it to e.g. C:\Games\AnimusRO, it works. If you don't know how to disable the user account control, go to google and search for "disable user account control".
Unless your user account doesn't have administrator privileges and the user account control is enabled, I have no idea why this is happening. @_@