Be careful about this. Some fall into a daze as they run defragmenting programs as their new main application.
For many years, people have written volumes about the disk file systems, what fragments quicker and how "bad" Microsoft file systems are.
My advice? Stop looking except once a week. Run it, then go play.
And be sure to turn off DMA and then back on because another Microsofft'ism shows it enabled, but it isn't. Here's how to turn it off and back on.
http://www.microsoft.com/whdc/hwdev/tech/storage/IDE-DMA.mspx
I am running XP Pro on a computer with a gb of RAM, and a 250gb drive that is partitioned into two volumes.
According to the Norton Utility sensor, my C: drive fragments by 30 to 40 per cent several times a day.
When I run Norton Speed Disk on Drive C: the defrag runs very slow, and doesn't get beyond 60% completed, and the disk is more fragmented than before.
When I run Norton Speed Disk on Drive D: (the second partition,) it defragments without a problem.
XP's disk defragmenter runs to completion, and the Norton Utility sensor says that my drive is 100% optimized.
Then, within hours, the drive is back to 60% or 70% optimized.
Norton Disk Doctor on Drive D:, behaves normally.
Norton Disk Doctor on Drive C:, generates the following message:
"The operating system, or another process, currently has exclusive access to this drive, or some of its files.
Norton Disk Doctor cannot continue with a repair under these conditions.
A repair can be scheduled.......
...Norton Disk Doctor can still diagnose this disk 'if' fix errors is unchecked."
When I run it without "fix errors" checked, it shows "Index Errors Not Fixed."
I run Norton Antivirus, and AVG antivirus (with current virus definitions,) several times a week.
I have had my "running processes" scanned and checked by the techs at HJTLOG at Siena University, and they see nothing unusual among my running processes.
I run ZoneAlarm Pro on my machine and closely monitor which programs can access the web. The fragmentation occurs, even when I have my DSL modem turned off, so I don't believe the computer has been hijacked.
Often, while I'm at the computer and doing nothing that would cause a "disk write," my drive will work, nonstop.
I have set the size of my paging file manually; I have let XP manage it.....I have even moved the paging file to my D: partion, all to no avail.
I've run out of things to try.........
Do any of you experts out there have any idea what's going on, and how to fix it?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic