I have a laptop running Vista Home Premium 32 bit. It's fairly new (~1 month) and has pretty much had this issue since day 1. If you leave the laptop idle for a "fairly long" period of time, it will lose connectivity to other machines on the local network. It will do it frequently overnight, but not always, so it's hard to pinpoint when/how long.
Here are the symptoms:
-Mapped drives still show up in explorer, but if you click on one, it hangs FOREVER. I have let it try to connect for several hours and it never gets through
-Using UNC gives the same result
-I can still ping other machines on my local network
-I can still access the internet
-Disabling/enabling the network connection does not help
-Once the issue starts, Windows explorer can not be shut down. You can task-manager-> end process all day and it will not die.
-Once the issue starts, Windows can not be shut down. It will hang on the "logging off" screen for literally hours. The only way to recover from this is to do a hard power down by holding the power button for 10 seconds or so.