Install Putty on windows client.

Get your private key from the AIX server, found in $HOME/.ssh/identity

Put that on Windows box.

Run puttygen.exe and load the private key, save it without password as identity.ppk in a safe directory.

To use SFTP use Putty's "psftp" as follows:

"c:\program files\putty\psftp.exe" -i \path\to\identity.ppk user@host