To set 256 colors and 640×480 resolution on XP:
Right-click your desktop
Select Settings tab
Select Adapter tab
Click List All Modes
Select 640×480 and 256 colors from the list
Do this before you launch the game. (Putting it in 640×480 resolution will run the game in full screen mode.) When you’re done playing, you can set your color depth and resolution back on the Settings tab (no need to go through all the steps).
Compatibility mode also has settings for 256 colors and 640×480 resolution, but it doesn’t work for many of the Sierra titles. To use compatibility mode, right-click on the .exe file, select Properties, and go to the Compatibility tab.