Why does disk defragmenter keep restarting/never finish?

Windows Defragmenter tool requires that no disk operation is occuring while it's shifting around the contents of your drive. Open applications generally access the drive quite a bit, causing the defragmention process to continually restart. (If you look, you should even notice a message on the defrag window indicating "disk contents changed, restarting...")
There are a couple simple ways to ensure defragmenter will not be interrupted:

1)Go into Control Panel, doubleclick on Display, and click on the ScreenSaver tab, make sure it's set to "(NONE)". [thx cdogg] Hit CTRL+ALT+DEL and note the list of programs running. There are only 2 items in this list that need to be active, EXPLORER and SYSTRAY. With each of the others, select and click the "End Task" button at the bottom. Once all but the 2 noted are closed, run defrag.

-OR-

2)Reboot the computer, hold down the f8 key while it rebooting, and at the Startup Menu select Safe Mode. Run defrag.