After the instance has been launched, from the EC2 console:
right-click on the Windows instance > Instance Settings > Get System Log
find the <Password>..</Password> tag and select and copy its content. This is the Administrator password, but ciphered with the Key-Pair you have selected when you first launched your instance.
Now, open a terminal window (i am using putty to ssh to my ubuntu server)
cd ~ nano EC2WindowsPassword.log paste the password here, save and exit base64 -d -i -w 0 ./EC2WindowsPassword.log | openssl rsautl -decrypt -inkey .ssh/Amazon-keyPair.pem > EC2Windows.Password
That’s it! Open EC2Windows.Password file and get your password.