Below I will outline how to automatically log into your Raspberry Pi from a Windows PC using Putty. This requires the setup of SSH keys, which is very easy to do.
1. Create SSH keys.
Download Puttygen from here and run it.
Once open, click on Generate and move your mouse around to generate your SSH keys.
2. Copy and past the public key, which is shown at the top of Puttygen into notepad, we will use this later.
3. Now click Save Private Key and save the key to your local drive
If you getting a warning about saving the key without a passprase, just click yes.
4. log into your Raspberry Pi and create the authorization file,
pi@raspberrypi ~ $sudo nano /home/pi/.ssh/authorized_keys
5. Copy in the text we placed into notepad earlier. If you already have a key in there, just paste the new one onto a new line. Then ALT-X to exit and save.
6. Open up Putty and add in the IP address for your Raspberry Pi and make sure that SSH is selected.
7. Within Putty, on the left hand side, expand connection and SSH and select Auth. In the path for Private Key for authentication select the the private key we saved earlier.
8. We now need to add the user name to log in with.
On the left hand side, select connection and then Data.
Enter pi as the username.
9. Go back to the main screen and save the session.
10. Now select Open to log into your Rasspberry Pi automatically. No need to enter username or password. The first time you log in you will be asked to accept the key.