XP Seems To Lose Network Connectivity To The Vista Computer

Have a small network consisting of one Win XP Media Center Ed. and one Vista home premium (and an Internet router). The Vista crashed and was restored to factory and apps reinstalled, etc. Each computer has dedicated IP address, and each runs a version of Norton Internet Security. They are in the same workgroup. Prior to the Vista problem, all was generall well. Now, the XP seems to lose network connectivity to the Vista computer when the computers are turned on in the morning. This is important for many reasons, including that the XP uses a printer attached to the Vista for most of the XP's printing needs.

I have set up a user on Vista that matches the user on XP, and have set the NIS on Vista to allow all connections to the IP of the XP box. And all is well until that reboot. There may be more to that, but in the few days the rebuilt Vista has been up, that has been the story. I have been able to restore the connection by launching Network Places, and searching for the name of the Vista computer. It so far does not show up in the network view until I do a manual search using the computer name. Then when it comes up and I double-click on the Vista C: drive, I'm prompted for login info, which I give and then I get connected (why is this not automatically done for me?). I've also mapped a drive to the Vista and set that for "reconnect at login."