I swear the safe mode might help, but I guess not. A program running in the background, such an anti-virus program has a tendency to interrupt the process because it is always keeping track of changes made on the hard drive. So before you defrag your hard drive you need to make sure that all potentially interfering programs are off. This is quite easily done. Anti-virus programs normally run from the System Tray. That's the little cluster of icons in the bottom right hand corner of the Windows screen. To figure out what does what roll your mouse over an icon and a little message box will pop up and tell you what the icon does. Once you have identified the anti-virus icon right click on it to display a menu. In the case of Norton Anti-Virus an option to "Disable Auto-Protect" will pop up. If you click on it will turn Norton Anti-Virus off.
There may be other applications that also get in the way of a successful defrag. You can turn any suspected program off in a similar way outlined above. Also be sure to turn your screensaver off. Go to Start, then Settings then Control Panel and double-click on Display, then the Screen Saver tab. In the pulldown box, select "None" and click Ok.