This warning is given typically when the Volume Shadow Copy Service (VSS) is unable to run due to another instance of VSS already running.
1. More than one backup program installed
If you have more than one backup program installed on your machine, disable all of the programs except for one and try run the backup job again.
2. Scheduling Conflict
Having two backup jobs running at the same time within BackupAssist both trying to run VSS can cause this error to occur. Please change the time one of the backup jobs is scheduled to start so that a conflict isn't likely to occur.
3. Previous VSS snapshot hasn't completed properly and is still running
Stop and Re-start the Volume Shadow Copy service. You can do this by going to Control Panel > Administrative>Tools > Services.
Once in Services, find Volume Shadow Copy and right click and select 'Restart'.
Also a reboot of the server has been known to clear this issue up as well.
4. Re-register .dll files associated with VSS service.
Open command prompt and change directory to windows/system32 and type in the following commands
regsvr32.exe /i eventcls.dll
regsvr32.exe /i swprv.dll
5. Apply hotfix to Server 2003.
An update is available that fixes various Volume Shadow Copy Service issues in Windows Server 2003