CPU/RAM problems with Minecraft server
1 gb is simply not enough for a server and your os together. Windows itself is using your RAM and there really isn't anything you can do about it.
Your server isn't using as much RAM as it is allocated because most of that RAM is being used by other programs.
Answering what you said in the comments below your post, the reason you could rent servers with 1gb or even less is because with those servers you get the full gb, with none of it being taken by the OS because of how they are set up.