Working at a clients I found that NTBackup, a backup utility that is built into windows operating systems (win2k-WinXP) has a problem backing up a computers system state, giving nothing more than an error code in the log file:

Backup StatusOperation: BackupActive backup destination: FileMedia name: "Nightly_Backup.bkf created 5/23/2007 at 1:23 PM"
Volume shadow copy creation: Attempt 1."Event Log Writer" has reported an error 0x800423f4. This is part of System State. The backup cannot continue.
Error returned while creating the volume shadow copy:800423f4Aborting Backup.
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The operation did not successfully complete.
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Doing some research I couldn't find the exact error number in Microsoft's vast database of higher knowledge, so I decided that since the error message said "part of System State" I started there and recreated the backups excluding the system state and so far that seems to be doing the trick...Thanks Microsoft for your ever mysterious error codes...