It should be in the readme...
"WINDOWED MODE
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To run The Sims(TM) 2 in windowed mode, first create a shortcut to the game by right-clicking on sims2ep9.exe and selecting "Create Shortcut." Then, right-click the shortcut and select Properties. At the end of the "target" line, add a space and the text "-w" (without quotes). Click OK, and then launch the game by double-clicking on the shortcut.
That's from the Read Me file that comes with the game. If you already have an icon on your desktop, you can skip the first part. So, just right-click on the icon, scroll down and click on Properties. Find the "target" line, add a spage and then type -w. Click on OK and then start your game up.
That's how it's done with all the other expansion packs. Have you already tried that and it doesn't work?