This is really, really easy to do actually. Find your latest EP icon on your desktop and right click on it. Select "Properties". Find the spot where it says "Target" and add a space and -w to the end of the target line. It should look like this: "C:\Program Files\EA GAMES\The Sims 2 FreeTime\TSBin\Sims2EP7.exe" -w
If you need a picture I would be happy to provide you with one. I would also like to mention that this information is contained in our
FAQ. It might be best to thoroughly read (or if you've read it before, re-read) our FAQ because it contains lots of useful information. And I bet I know why your search didn't work; it's not called "windows mode", it's called "windowed mode".
ETA: Whoops, fatkitty beat me to it. I'll let my post stand because the additional information I provided (FAQ link, windows/windowed explination, target line example) may be useful to someone, somewhere, someday.