First of all: make sure that on the Windows machine the Volume Shadow Copy Service (VSS) is not disabled.
Go to Administration Tools > Services and check if the service is disabled and, in case it is, enable it.
If this will not solve the issue it's maybe a permission problem and you have to modify some registry keys on the Windows machine to grant access to the user used by Bacula.
Open regedit and navigate to the key HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/VSS and add the key VssAccessControl if it doesn't exist.
Then add a value that is the name of the user and set it to 1.
Its name have to be in the form domain\username.
If you keep on having this issue check the registry log on Windows machine and follow this ugly guide from Microsoft: http://technet.microsoft.com/en-us/l...35(WS.10).aspx