Zophor If you click ctrl-alt-delete, can you choose Sign Out? That would boot you to the Windows login screen and close the game in the process.
If that doesn't work, and you're playing in fullscreen mode, consider using windowed mode instead. If you don't like the look, borderless mode works like windowed mode but looks like fullscreen. The Smooth Patch from Lazy Duchess has a Borderless option, and Windowed Borderless Gaming is a free third-party tool that works well with Sims 3. Either option should be fine.