I just had this problem (with a newer version of Git, 1.7.9). I used the answer from VonC, but only a couple of steps were needed:
- Set the environment variable
GIT_SSHto the location ofplink.exe - Run
pageant.exeand load your private key
On 64 bit Windows, the default location for plink is C:\Program Files (x86)\GitExtensions\PuTTY\plink.exe
The spaces in the path no longer seem to be a problem.