How to reactivate GRUB

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http://askubuntu.com – Have a computer with two Ubuntu versions on it in separate partitions. For the fun of it, I installed WINXP Home in a thrird partition. I then deleted WINXP but now I don't get the GRUB boot menu. Is there a way to get the GRUB boot menu back without reinstalling one of the UBUNTU versions already on the hard drive? (HowTos)