Mapping a network drive with different credentials?

I have a routed home network; two computers are located on a wired network in
the 192.168.0.0 network, and three (my computer, along with two wireless
notebooks) are on the 192.168.126.0 network, connecting through a D-Link
DI-524 router which is connected to the 192.168.0.0 network. This network is
a workgroup, and not a Windows domain, and the connection to the Internet is
provided through the 192.168.0.0 network.

At 192.168.0.254 is a computer running Red Hat Linux serving as a file
server. This workstation is only used as a file server, and we store our
system backup files over the network to this machine, which in turns stores
the backups onto a tape backup.

I installed a fresh copy of Vista Beta 2 (32bit) last night on a clean
drive, and attempted to access the network share to retrieve my backed up
word and excel documents.

When I try to map a network drive to the 192.168.0.254, I don't see any
options to use a different login to connect to the network share. When
running the wizard the share is located, and the wizard "finishes" without
any sort of prompt for a username, and the network drive does not appear
under Explorer. And using net use Y: \\192.168.0.254\backup /user:<username>
<password> didn't work, while the command functions fine under XP.

My hypothesis is that there's a setting I need to change to "trust" the
192.168.0.x network; unfortunately I haven't been able to find it yet. Any
suggestions would be appreciated.