Is that a command specific to XP home or something?
Net use does not allow you to specify a password - it has to be typed manually, after the net use has run. You can create a VB script to get around this but the net command alone will not work.
The guest account issue is due to a local policy restricting access to network resources. The same policy also disables the sharing tab in folder properties.
So to clarify, the guest account in the windows logon window has nothing at-all to do with the DNS-323.
Pass-through may well work - in which case the net use command without the password would likely work. I.e. if you logon to your XP devise using joe and password blogs - create an account on the nas called joe with the password blogs and simply use 'net use * \\nas\share' and it should work.
You can replace * with a drive letter if you want too - net use x: \\nas\share, for example.