VPN drops using VNC and XP

When using VNC on an XP host machine to connnect to a remote Vista machine, I can make an initial connection, but the VNC window freezes after several minutes of inactivity. If I continue to keep the machine active, then my connection holds up. Similarly, I have spent time connected to the machine while another user gives a presentation then my connection holds up. I have also found that if I disable the my Windows Firewall, then I can leave the VNC window connected and inactive for several minutes without losing my connection--with the Firewall on, the connection will fail during the same period. Has anybody experienced this? suggestions?

Thanks,
Chris

My machine:
Dell Latitude
CPU Intel T2300 (1.66 GHz)
XP SP2 using windows VPN connection
VNC Viewer Personal Edition P4.4.3(r16583)