Did you know that sometimes when microsoft automatically applies its updates to your PC, it will reboot your PC without asking or informing you.

If you have set microsoft updates to automatically apply updates - make sure you have set it to run at a different time than your backup.  Restarting your PC in the middle of a backup is not advised.

To check your microsoft updates settings (in XP) open the control panel, click on the Security Centre Icon, and click on the words 'Automatic Updates' in the Manage Security Settings section at at the bottom of the window.  Change the date or time as needed.