Microsoft System Center Configuration Manager 2007 backups may fail if other VSS-aware backup software is scheduled to run concurrently.  In this scenario, the Smsbkup.log will show entries similar to the following:

Deleting E:SMSbackupSiteServerSMSServerinboxesauthstatesys.boxincoming, FAILED, Win32 Error = 145
Deleting E:SMSBackupSiteServerSMSServerinboxesauthddm.box, FAILED, Win32 Error = 145
Deleting E:SCCMBackupBackup, FAILED, Win32 Error = 145
Failed to delete the contents of the backup folder. Error Code = 0x0 SMS_SITE_BACKUP
Error: Deleting the existing files in the backup location failed… SMS_SITE_BACKUP
SMS site backup failed. Please see previous errors. SMS_SITE_BACKUP
SMS site backup service is stopping. SMS_SITE_BACKU

These errors can occur if the Volume Shadow Copy Service (VSS) provider is in use by another process.  Configuration Manager Backup expects to have exclusive access to these resources when performing a backup.

To prevent these errors, configure Configuration Manager Backup to start at a time outside the window for other backup jobs on the server.  The schedule for this can be found in the Configuration Manager Console under Site Management -> Site Code -> Site Settings -> Site Maintenance -> Tasks -> Backup ConfigMgr Site Server.