Vista drops XP on network map

Sorry if this is a repeat - I don't see my post from last night...

My home network consists of a Windows XP Home SP2 machine and a Vista Home
Premium machine. Both machines connect to the internet via a cable/dsl router
connected to a cable modem. There is a shared printer that hangs off of the
Vista machine. LLTD protocol has been installed on XP and netBIOS over TCP/IP
has been set on Vista. XP is running the Windows firewall; Norton Internet
Security is installed on Vista with the Windows firewall set to off.

Upon initial boot of Vista both computers are able to see each other, files
can be transferred and print jobs are successful. The problem is that the
connection eventually disappears from the Vista network map. When this
happens, neither machine can see the other and files are unable to be
transferred. However, XP can still print. This problem will remain until
Vista is rebooted.

Any ideas/suggestions?