bootloader problem: no dual-boot of fedora 17 and Win7

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http://forums.fedoraforum.org – Hello! I installed fedora on a PC with Win7 on it from live-usb and had an error: "There was an error installing the bootloader. The system may not be bootable. " I rebooted and Win7 startet without a choise. Then I tried to install Fedora again this time I choose to install grub at the fedora partition and it worked without error, but the restart started Win7 again. I started Live-Usb and gparted and set boot-flag to the linux partition and fedora started with grub, but chosing Win7 couses an error. I can start Win7 by setting the boot-flag to the Win7 start partition. I tried (HowTos)