Windows 7: Hard Drive with MBR Failed; how do I get main installation working?

Try it yourself. Safest thing to do:

• Mark the current ACTIVE partition INACTIVE
• Do a startup repair

It's rarely an issue where no partition is marked Active. In fact it isn't until toward the end of Troubleshooting Windows 7 Failure to Start that a step is given to check the Active flag is on SysReserved (preferred) or C before running 3 Repairs, then move it to the other to try 3 repairs again - all a last resort.

In rare cases if something is blocking repair of Active partition this can be circumvented by writing the System boot files to another partition - even a freshly constructed SysReserved partition - using Startup Repair x3 - all last resorts.