Monty,
There are a ton of threads that have been started in the past on this. I highly suggest searching for common questions using the advanced search option above.
In short, defragmenter can become interupted if something running in the background makes a "disk write" to the hard drive. Screen savers, power management, antivirus scanners, etc. are all examples of interference. You can try to kill them before hand by hitting CTRL-ALT-DEL and ending task on everything except Explorer (don't worry about the "killing" part - rebooting will bring them back).
If your friend can't determine where the conflict lies, have him defrag in safe mode...
~cdogg
"The secret to creativity is knowing how to hide your sources."
- A. Einstein