system

Installation from nothing : First win7 then linux-mint.
Win7 writes in MBR something to control boot and linux uses grub to control boot which will be able to boot windows.
Why not in the reverse order?
A complete re-installtion of win7 may over write ALL in MBR meaning the part that will call the actual grub configuration (in /boot or /etc/grub.d not sure...), therefore the repair of the linux booting is needed.
Result: grub boot first, if you choose "windows"...