Since you have Grub installed to the MBR, you're safe.
as bcbc stated: "It's safe to format the windows partition (assuming you installed the grub bootloader, which is the default).If the bootloader had not been installed, at that stage your computer would be unbootable, Windows bootloader in MBR pointing to a deleted partition. @ that point, if you didn't have a liveCD/USB, you could take a break until you got one, then boot and fix the MBR, "install Grub".
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