Assuming that the malware has already been removed, you will need to purge the Volume Shadow Copies before you will be able to perform the backup.
Clear Volume Shadow Copies on Windows 7:
1. Click Start > Computer.
2. Right click on Local Disk C (C: ) and select Properties.
3. Accept any alerts from Windows UAC.
4. Click the Disk Cleanup button.
5. Wait for Disk cleanup to finish calculating.
6. Select the More Options tab in the new window.
7. Select the Clean up button for System Restore and Shadow Copies.
8. Click Delete to confirm.