How to Disable Sleep Mode in Vista
- 1). Click the Start button on your task bar, and type "Power Options" in the search menu.
- 2). Click to open the Power Options dialog, and press Change Plan Settings for the selected profile. The profile you currently use will have the radio button next to its name filled.
- 3). Click on the drop-down menu next to the Put the Computer to Sleep label. Select Never and press the Save Changes button to apply your customizations. Your computer will no longer enter sleep mode.
Source...