Vista will defrag when you're not doing anything. Set the machine to sleep/hibernate automatically and it will defrag when it makes sense to do so. You won't run into a situation where a defrag is slowing down your work. And if the machine is sleeping during the regularly scheduled defrag time, it will re-schedule the defrag to a different time and make up for it at some point. The beauty of Vista defrag is that you can completley ignore it and not think about it and it gets the job done effortlessly. People are obsessed with defragging for some strange reason and will recommend you install third party utils and such, but I suggest you ignore that advice, ignore defrag, use your computer normally and save the time and energy for something more interesting and useful.