You'll need a Win7 Installation DVD for your licensed version or a System Repair Disk.
You can google the official Win7 ISO download for your version to burn to DVD using ImgBurn, to use for Repairs or Reinstall.
Boot into the DVD or Repair CD System Recovery Options
to mark the Win7 partition Active: Partition - Mark as Active (Method Two)
Then also from DVD or Repair CD run Startup Repair - Run 3 Separate Times
until Win7 starts on its own and holds the System Active flags.
Once you get to Win7 run Belarc Advisor
to audit the Product Key for either or both OS's as you will need this for reinstalling or even some repairs. You can lose the value of the OS if you don't have these keys to back them up.