Puttygen not a private key

PuTTY does not natively support the private key format generated. Fortunately, PuTTY has a tool called PuTTYgen, which can convert keys to the required PuTTY format.
convert your private key
Start PuTTYgen (e.g., from the Start menu, click All Programs > PuTTY > PuTTYgen).
Click Load and browse to the location of the private key file that you want to convert (keyname.pem). By default, PuTTYgen displays only files with extension .ppk; you'll need to change that to display files of all types in order to see your .pem key file. The private key file must end with a newline character or PuTTYgen cannot load it correctly.

Select your .pem key file and click Open.
PuTTYgen displays a message.

When you click OK, PuTTYgen displays a dialog box with information about the key you loaded, such as the public key and the fingerprint.