Often, when you use windows you will have to format you system because of virus and spyware.
If you are using linux in dual boot with windows, then, grub will be erased and you will not be able to boot to linux.
Fortunately, it is easy to reinstall grub if such situation happens. Just follow these steps:
1. Boot with the installation media (CD/DVD)
2. Choose rescue existing system
3. The system existing will be mounted on /mnt/sysimage
then, execute the following commands
assuming the your fedora system is installed on /dev/sda. You can check this with fdisk -l.