Dual-boot problem (Windows Vista doesn't boot)

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http://forums.fedoraforum.org – Installed Fedora 16 x86_64 LXDE a few months ago. Before installation I resized my C:-partition from inside of Windows so I could get some free space to install Fedora on (so I would be able to dual-boot with Fedora and Windows Vista). The installation went fine and Fedora is usable and all BUT I am not able to boot Windows Vista anymore! If I choose my Windows-partition from the GRUB-menu Windows starts booting but then suddenly the machines just reboots itself and I'm back at the GRUB-menu! What can I do to get Windows working again? (I'm able to access my Windows-files from within (HowTos)