Are you running Microsoft Security Essentials by chance?
If you are, then uninstall it and run the back up again, should work fine. If not then follow the steps below
You must be logged on as an administrator.
1. Open Administrative Tools by clicking the Start button, clicking Control Panel, clicking System and Maintenance, and then clicking Administrative Tools.
2. Double-click Services. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
3. If the SPP service is disabled, double-click it to open the service properties.
4. If Volume Shadow Copy service is double-click it to open the service properties
5. Click the Startup type list, click Automatic (Delayed Start), and then click Apply.
6. Under Service status, click Start.