There can be 2 Scenarios here:
1. You are trying backup after the installation of the new OS:
You may just restore the backup on the newely installed OS or use Acronis.
2. You are trying to Rebuild the MBR with a Windows 7 CD which is already modified by the Symantec Encryption.
No you cant use the windows 7 disk to rebuild the MBR .Also rebuilding the MBR wont change the whole Drive Encryption State.