The private key that's on your Windows 7 computer should never leave that computer.
Create a new private/public key pair on your Mac, and add the public key to the server you want to connect to (i.e. your hosting company's server which has the git repository on it). If the server is a Linux server, you can just append the contents of the new public key file to ~/.ssh/authorized_keys
on the server. For other kinds of servers, there will be some equivalent way to add the new public key, but I don't know what it would be - it depends on the server OS and possibly what SSH software it runs.