Some users make the unfortunate assumption that the network somehow represents an efficient and organized bureaucracy worthy of their trust. Far from the truth. Never assume that the network jocks are doing their jobs backing up the network data every day, even if they are. Check up on them. Conduct a surprise inspection one day: Burst into the computer room wearing white gloves and demand to see the backup tapes. Check the tape rotation to make sure that more than one day's worth of backups is available.
If you're not impressed with your network's backup procedures, take it upon yourself to make sure that you never lose any of your data. Back up your most valued files to a CD-RW drive or a flash drive.