Should I stop all work on my computer while it's being defragged?

Generally, yes, especially pre Windows XP. The reason is that on older OSes the program monitors disk activity and if it monitors ANY kind of change to the disk it has to go back and re-analyze it again so you'll get this irritating loop where it'll defrag for a second then go back and re-analyze for a minute, then defrag for a second and go back to analyzing... XP doesn't seem to have this problem.

Also on older OSes you'll really want to end as many applications as possible before defragging since many of them will commit write operations while the defrag is in process, also leading to the endless loop. I found many times I had to disable antivirus or quicktime or other things that were constantly doing operations and writing to disk. It get very aggravating. I usually just close everything, run defrag and just walk away for 1/2 hr or so. Though underXP I just don't worry about it since it doesn't have the same issues as older OSes in my experience. So, on XP, surf away, as long as your machien has a decent processor, fast HD, and a decent amount of memory so it's not using he HD for a big virtual memory scratch disk, which would tend to bog the system down if the HD is constantly being accessd by the defragger thus not allowing the HD to be sufficiently used by windows for virtual memory.

Also, you should probably do any disk cleanup before defragging to get the maximum amount of defragged free space. Things like clearing Internet explorer's cache, emptying the recycle bin, etc.

Yes, when you are working on your computer the memory is swapped out to a swap file on the specified hard drive when more memory is required. When the memory swapped to the hard drive is accessed, the current memory is then swapped with the memory on the disk.

When this happens during defrag, the defrag operation sometimes must stop then start over due to the contents of the disk being defragged changing during the defrag operation. .

Yes. It's counter-productive to use the same harddrive or volume while defragging it. Also, it's very laggy.

Set you computer to defrag sometime when you are at work or over night. Defragging usually takes a whil, if you got a decent sized harddrive that's decently fragged.

Yes, unless you want it to run about 10 times as long. When I got my first computer I didn't understand why defrag was taking so long. Then I found out I'd forgotten to stop my screen saver. It kept having to re-read the drive, over and over again.

Michael Gmirkin's response is quite good. I'll add that the defragger he's talking about isn't just the XP defragger, it's the whole NT-2000-XP series defragger. If you have Windows 95/98/Me the defragger will stop at anything, but the NT-2000-XP one is more robust. You can even keep working with that one, although you won't make any real progress if you're using virtual memory (the swap file) at the same time.

Yes close all windows ... do not serf stay on that page

YES, definitely YES!

depends what cpu you have, if it is strong enough then no.