Backup Utility Problem

When using Backup Utility for Windows (ntbackup.exe) I have suddenly
encountered this problem:
1) 'Backup' (normal, incremental, copy or differential) failus
completly.
2) 'Restore' (from preciously created archives) works fine.
Note:
1) I am Not referring to the 'System Restore Application'.
2) The backup Backup Utility for Windows previously worked flawlessly.
3) My system is otherwise working perfectly.
Windows XP Pro + SP2
GHz Athalon 64
1 Gb Ram
Virus scans (AVG + Norton) -> Negative
Spyware/Malware scans (AdAware + Spybot) -> Negative

Addendum to 'Backup Utility Problem'
When backup fails I get the following error message:

Operation: Backup
Active backup destination: File
name: "Backup.bkf created 12/26/2005 at 12:20 PM"
Error returned while creating the volume shadow copy:80042301
Aborting Backup.
The operation did not successfully complete.

------