Snapshots created for a virtual machine on an ESX host, may not be deleted after a backup if the remote agent crashes abnormally during the backup operation. So that such orphaned backup exec snapshots can be deleted automatically, following registry key could be set:
HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Backup Exec For Windows\Backup Exec\Engine\VMware Agent
1 = Orphaned snapshots created by Backup Exec are deleted at the beginning of the backup.
More information here: http://www.symantec.com/business/support/index?page=content&id=TECH200709