Well yeah, anytime you run a defrag, you shouldn't be using your computer. This is coming from my computer programmer.
Anyway, I often forget to defrag my hard drive and right now my hard drive is dying so it would be a bad idea to defrag it right now. I'll wait to screw around with it until my new one comes in the mail so I'm not without my desktop computer. Although, I have a few laptops lying around here it's much for fun to do SEO on a desktop!