I have 2 servers running SQL Server 2005. One has Standard and the other has Express. Both are running on Server 2003 w/SP1.
Since I installed SQL 2005, Backup Exec (through Remote Agent) has been unable to backup VSS on either computer most of the time, although it's occasionally worked for a day or two but usually not on both servers at the same time. When I check the event log there are COM errors starting VSS (which I have set to start automatically) and VSS is not only stopped, but actually disabled. I installed the hotfix from MS KB891957 and also ran these commands to re-register several DLLs and rebooted:
Net stop vss
Net stop swprv
regsvr32 /i swprv.dll
regsvr32 /i eventcls.dll
Net start swprv
Net start vss
After those steps I found a reference to a problem that seems to be created when SQL 2005 installs the SQL Server VSS Writer. Apparently it screws up the COM+ catalog, which I was able to verify from the command line by using the vssadmin list writers command (nothing was listed). In order to fix that I deleted the HKLMSoftwareMicrosoftCOM3 registry key, booted to the Recovery Console, and renamed C:WindowsSystem32clbcatq.dll to ~clbcatq.dll. Then I started Windows and went into Add/Remove Windows Components and clicked Next so COM+ would be reinstalled and the catalog rebuilt. The VSS writers came back, but when the databases were backed up (using T-SQL's BACKUP) the SQL Server VSS writer stayed in state 5 - Waiting For Completion, and as a result when Backup Exec tried to do it's VSS backup later it caused VSS to fail several times and eventually it got disabled again.
Finally, I found the registry key that tells SQL Server whether to use the new VSS Writer or the MSDEWriter for backups (HKLMSYSTEMCurrentControlSetServicesVSSSettings) and set MSDEVersionChecking to 0, which is supposed to make SQL Server default to MSDEWriter. I also disabled the SQL Server VSS Writer service. This morning when I checked the backup VSS was disabled again and so was the MS Shadow Copy Provider service. When I enabled them and listed the writers again SQL Server VSS writer was in state 5 and of course the VSS backup failed. After re-enabling VSS and MS Software Shadow Copy Provider, as well as disabling the VSS writer again I ran the database backups followed by the Backup Exec job that backs up VSS. The database backups used the SQL VSS Writer again but this time they completed (VSS Writer state was 1 in vssadmin) and the Backup Exec job completed as well.