First Post (be easy)
I have several linux guests on an ESXi server. I am using UPS monitoring on a physical windows box and would like to be able to shutdown the linux guests from a batch file on the windows box. I found a way that should do it using putty/ssh and private keys but I haven't found a way to create a user on the linux guests without a password, hence the putty session hangs asking for a password. None of the linux guests have outside internet access from the firewall and are all on an internal vlan.

Anyone have any ideas?

But if you use private keys no password should be needed.
* Can you login on a Linux via putty without password?
If not, you haven't got the configuration for using keypairs right.
If you can, there's something wrong in your script.

One other possibility: your user on Linux-box is not allowed to shutdown the system. Normally only root can do that.
May I guess that you are trying to script a sudo-command? Something like 'ssh user@server "sudo poweroff"' ?

To create a user without password, follow the steps. (Note here i'm using username dummy)

1. Become root
   su
   
   
2. Create user account
   useradd dummy
   
   
3. Unlock user account password
   passwd -uf dummy
   
   
4. Done. now you can login as user dummy without password
   su - dummy