Acronis does not remove it's VSS writer registry key when uninstalled.
1. Start > Run > Regedit and delete the registry Key for the Acronis provider. HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\VSS\Providers
2. Open each key to view it's contents. Locate the Acronis key.
3. Right click the Acronis key and export the key in case you need to restore it.
4. Delete the key.
5. Restart the VSS service if necessary.