How can I restore grub after switching from Windows 8 to 7 on a dual boot?

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http://askubuntu.com – I formerly had windows 7 without ubuntu. Now I upgraded it to windows 8..then I installed ubuntu 12.10 with live disk. After installing it runs ok and I see a dual boot option in grub menu. Now sometimes I log in to windows 8 for some tasks. After some days I find windows 8 not interesting and I want to reinstall windows 7...now to do that I have to boot windows 7 disc from my dvd drive just like before.. My problem is if I want to reinstall windows it will not again show the grub menu and will directly go the windows desktop... So how can I get back that boot option menu (grub) to log in (HowTos)