If you're using Win98, or one of the later Windows versions with FAT32 disk format, the "something" that's writing to disk is probably the defrag program itself. It's a "built-in" characteristic of the utility.
There were several threads relating to the "defrag problem" a couple of years ago, and lots of suggestions (and complaints), but the simple fact is that a scandisk or defrag with some system/disk format combinations may take a very long time to complete, and may look like all that's happening is that the first 10% of the disk keeps "starting over." The utility is running and is doing what it's supposed to do. It's "passing the buck" to make you think that something else is interfering, but in most cases it is the defrag utility itself (or scandisk) that is actually doing the "writing to disk."
It does make sense to turn off any program that may "auto start" during a defrag. Screen savers should probably be turned off, and if you have a web connection that pops up it could interfere. Turning off systray or even doing a "safe boot" to get a super-clean machine for a defrage will have little real effect.
If needed, you can safely "interrupt" a defrag, and continue with it later; but my "personal record" for a defrag with Win98 on a FAT32 drive was over 70 hours, and some people have reported longer times. Let it run as long as you can, and restart it when you can let it run some more. It will eventually finish the job.
If you've turned off the screen saver, and any other programs that are realistically known to jump up and run then it's largely a matter of letting the utilities do their thing - until they're done.