Check the power management settings to ensure that network activity prevents the computer from sleeping. In addition, go through all the currently running software to see if there is some 3rd party application that handles power management itself.
If you are still having problems, try setting it to "Always On" when plugged in.
If it still disconnects when the screen saver goes on, try disabling the screen saver and just using the energy saver/blank screen.
Welcome to Vista
