You can't encrypt a password, but you can use a SSH key file instead.
There is a tool called puttygen which can be used to generate a key file.
Puttygen will generate two files.
- Private key
- Public key
The public key needs to be added in the server under $HOME/.ssh/authorized_keys.
After adding the public key it is necessary to attach the private key in Putty.
Find SSH in the left menu SSH->Auth-> Browse and add the file.
You can also store this as a session in putty so that you don't need to attach each and every time.