Postgresql: When using SSH tunnel, I receive the fe_sendauth: no password supplied error
Your Windows 7 PC supports IPv6 natively.

I'd say your pg_hba.conf is set to require md5 on connections from ::1 but not on 127.0.0.1. This works on your XP machines because they won't connect over IPv6. On Win 7, they'll happily use IPv6 (and PuTTY will happily tunnel IPv6), so they probably appear to have the origin address of ::1.

To confirm if this is the case, look at the PostgreSQL log file on the server.

(Personally I find it much less of a pain to just configure SSL on my PostgreSQL servers anyway, mind).