Your system has taken a 'system snapshot', to facilitate a system restore. The snapshot includes the infected file, but Windows is locking the snapshot (to prevent over-writing), so the AV programme cannot access it to deal with it.
If you disable system restore, reboot, then re-enable system restore, all snapshots will be deleted. This will clear the infected file, but will obviously prevent you from restoring to a point before when you disabled system restore.
Windows Help will show you how to disable / enable System restore.