Did you convert the keypair with puttygen? This the best EC2 getting started guide. See the appendix on using Putty.
I’ve noticed that this step has become unnecessary using ElasticFox, if you create keypair with Elasticfox, and use that keypair to start an instance, then right-click the instance and choose connect to Public DNS name – which opens putty up and logs on, you don’t even have to type “root”
You have to set the path to Putty.exe, click the Toolt icon in Elastic Fox (upper right next to About) and enter full path to Putty.exe in the SSH Command box.
Richard