Download Putty and Puttygen from here: http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html
(You only need to do this once!)
Open puttygen; select “Load”.
Find and load your ‘.pem’ file; it’s probably in your Downloads folder. Note, you have to select ‘All files’ on the bottom.
Now, “save private key”. Put it somewhere easy to find.
Open up putty, and enter your hostname into the Host Name box.
Now, go find the ‘SSH’ section and enter your ppk file (generated above by puttygen). Then select ‘Open’.
Log in as “root”.