If you want use your existing windows installation , you follow the METHOD 2
or METHOD 3
of the tutorial Partition - Mark as Active
,to mark your windows 7 's C partition as active, and then run Startup Repair
for at least three times, with restarts after every single run. Later, you can add linux boot entry using EasyBCD
if you want to reinstall windows, follow the tutorial SSD / HDD : Optimize for Windows Reinstallation
to clean the disk from GRUB and EFI partitions. But before that, dont forget to backup all your important data externally.