When you launch an EC2 instance, you assign to it a key pair (or none). It cannot be changed afterwards.
Only using that .pem file from that key pair will you be able to decrypt the Windows password.
The .pem file would have been downloaded when the key pair was created. You cannot get it again. If you have lost it, you're out of luck.
You cannot use a .pem file that you generated yourself unless you imported that key into AWS before the instance was launched and assigned it to the instance.
Simply put, if you don't have the original .pem file, you cannot get the password.
Edit: after re-reading the question, I realize the OP had imported his key into AWS.