Defrag won't finish

A friend of mine (who lives a long way from me) can't get defrag to complete.
Each time he runs it when it gets to around 60,000 it stops and restarts.
I have suggested he does a 'disk cleanup' first. I feel that a program must be writing data to the disk (I can't watch what is happening) which is causing defrag to stop and restart.
Is there anything else which could be causing this? Is there anyway to get around this apart from booting to DOS (bypassing Windows) and running defrag from there?
Thanks
John

 

johngarnold--Your advice to do a disk cleanup first is good. Your friend should also run ScanDisk before running Defragmenter.
One of the most common causes for Defragmenter stopping and going back to the beginning is that some other program has "written to the hard drive ". This could be the screen saver, a ping from an internet connection, a Scheduled Task that has come to life in the middle of the defragmenting, etc. To avoid that, all programs should be shut down except for Systray and Explorer. Easiest way is to hold down Control+Alt+Delete simultaneously and highlight and End Task on all other programs shown as running before running both ScanDisk and Defragmenter.
Of course, there could be something corrupt with the file when the restrart occurs, but try the above. (There is nothing to lose and ScanDisk may also have fixed the corrupt file.)