If it's been a very long time since you ran defrag, a rerun sometimes can sort out additional remaining fragmentation. WinXP probably will tell you that you don't need to run it, but you can tell it to "run anyway."

A first pass at defrag usually assembles most of the files, but may not consolidate the free space. A second (or third etc) rerun should move all the files together to consolidate the free space into fewer but larger empty spaces. This can affect how the OS assigns and uses "temp files" and sometimes helps with how efficiently some programs run. The main advantage though is that with all the free spaces assembled into a few bigger empty clumps the system doesn't build up lots of newly fragmented files quite as rapidly as if new files have to be poked into many smaller spaces.

A problem with rerunning defrag more than once is that frequently the "closer you get" to perfection the longer it takes to run the next pass. My notes show that with Win98SE in one "determined effort" to get as clean as possible, the first defrag was about 3 hours, the second took about 5, and after a couple of additional passes the "peak" pass took about 17 hours, after which the last one - with all the free space pretty much consolidated - was back down to under two hours.

To "reassemble" a file only one or two files may need to be moved, but to merge a couple of free areas, all, or nearly all, of the files in one clump have to be moved over next to the other clump, so it does take longer to shove everything together than just to put all the individual files together.

This isn't something that's really "necessary" with WinXP and later, since the newer Op Systems are more flexible about using fragmented free space. You might consider it if you have a few hours when you can leave the machine on but don't really need to use it; but it's definitely "optional."

It was sometimes a really big help with Win98 & WinMe (or even older systems), especially if you had a nearly full drive.

Defrag almost never needs to be run manually in Vista or later, since the OS runs it in background automatically whenever more than a little fragmentation appears.

If you feel inclined to try another defrag pass, be assured that you can interrupt defrag at any time with no harm to the system. It will start over the next time you start it, but what got done will be done.

Incidentally, in Windows Explorer you can right click on a Drive and select "Properties." There should be a "Disk Cleanup" button there that's a little more convenient to find than going the Programs|Accessories|System Tools route to it. When running Disk Cleanup, it's significant to remember that you cannot delete a file that is open, and neither can Disk Cleanup. You need to run it with the absolute minimum number of programs turned on to get a full cleaning.

John