Download Putty and Puttygen from here: http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html
(You only need to do this once for each key!)
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.
Load it.
Now, “save private key”. Put it somewhere easy to find.
Now that you’ve generated your PPK file from your PEM file, you can log in. To do that...
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”.
Declare victory!