Question - what backup software are you using?
At this time, I can only suggest that it may be a result of one three things:
1) Corrupted data source (CD-RW, flash memory, or whatever)
2) Malfunctioning backup software
3) A slight change in Windows configuration or installed software - for example, using the XP File and Settings Transfer Wizard as a means of backup/restore does not work properly if you haven't loaded the same software. The settings file will, in this instance, fail to load.