IronLinux.org

Windows is not really friendly towards other operating systems - when you installed it while another operating system is already installed, it will replace the bootloader with its own, without including any reference to that other operating system - preventing you from booting it. This is why it is often recommended to install Xubuntu after Windows. Sometimes, however, installed Windows afterwards is unavoidable. Restoring Xubuntu’s bootloader (GRUB) can be a pain.

Luckily, David Mooney comes to the rescue: in a post to the xubuntu-users mailinglist, he explains how to restore GRUB:

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