You can, but it is not a smart idea. You HDD moves around a lot when defragging. It also does that when you are playing games. It's like stepping on the accelerator and brake pedals at the same time on your car, sooner or later something is going to give. Doing both at once can cause your HDD to overheat, and fail. (BACKUP, BACKUP, BACKUP) You can, however, browse the web safely, or use any application that doesn't access the hard drive too much. You can test this by looking at the HDD light on your computer case/ laptop. This tells you when the hard drive is being accessed. (When you first launch an application, no matter what it is, it will access the HDD a lot. Also, while defragging, this light will stay on, that is normal. Generally a kirr-kirr-kirr is a good thing while defragging. Genreally a KIRR-DUK-DUK-KIRR is not a good thing while defragging. Mister El has no clue what he is talking about, he is speaking out of his ***. Try Auslogics Disk Defrag: http://www.auslogics.com/disk-defrag/index.php It is much faster than the built in utility included in Windows.
Hope this helps.