Hi Oscar,
It may be that you have an app that is accessing the disk ocasionally
running in the background, if this is the case everytime it accesses the
disk, defrag will restart as the disk has changed state and you will be
stuck in an endless loop while it tries to get the state of the hard disk
consistent, ie defrag starts, app modifies disk, defreag restarts, app
modifies disk, defrag restarts etc.
First check there are no other apps runing in the task bar,
Exit out of any apps that aren't necessary in the system tray,
Try Ctrl+Alt + del, and see what apps you have running in the background, if
any of them can be stopped stop them and this should cure your problem.
HTH
Joe