Also, we are talking about Windows here, right? Even Windows 7 needs to be rebooted time to time. The OS is usually fine...it's 3rd party applications that have nasty things like unimplemented traps or memory leaks. One or two services like this can bring your performance to its knees right up until the OS decides to kill the service to free up resources. This used to be a lot worse in earlier versions of Windows, but bad code is still evident today, even in commercial applications.
Outside of memory issues (or lack thereof), you would be hard-pressed to find any quad-or-more-core CPUS that can't cut the muster for everyday office tasks like Word, email, web browsing, video and music, no matter how many windows you have open. In fact, I would say that you would bounce against memory constraints long before you choke the processor with such tasks.
Now, if you were trying to encode video and/or music while you were opening a 200+ MB Illustrator or InDesign file while also resizing a catalog of high-res photos using a Photoshop batch script while also burning a DVD...you might notice a slowdown. I do this on a regular basis. And that's not for work. My day job is UI programming, design and development.