"Logon Failure" when running executables on NAS

I have a Linux-based NAS running Samba with user-level password
protection (the same userid and password as Vista). I can browse files,
etc., but I am unable to run any executable files. UAC prompts me for
approval, but then give me a "Login failure: unknown username or bad
password" error. It must be a UAC issue because I have no problems when
I turn UAC off, but I'd like to leave it enabled if possible. Anyone
else experience this issue and have a solution? What exactly is Vista
sending as the username and password when elevating an application via
UAC? Maybe I could set up a dummy user on my NAS that reflects the user
and password Vista is sending.

Thanks.