mapping network drive 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.