You might try installing from the command line with a command like this:
*msiexec* /i pgadmin3.msi *ALLUSERS*=""
That should run the installer but for it into per-user mode.
From: http://www.postgresql.org/message-id/BANLkTimHUFk9CZQKigHcvFd1Om39nfnKnA@mail.gmail.com