windows reinstall issues

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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... 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 to (HowTos)