The Master Boot Record is erased after a Windows reinstall on a dual-boot. this is the method to recover GRUB in ubuntu and fedora core...
Fedora Core
Boot with a live cd that has grub. Issue these:
logon as root/type su in the terminal/konsole
(? being where your controlling distro is)
Alternative method
login as root/use su
and then it will tell you that the installation is completed witout errors and you can reboot your machine
it can be done by any of the above methods.
Ubuntu
1.)Boot from the Live CD.
2.)Open the terminal. Quote:
this opens the grub prompt
this command gives a location (or two)
this will inform u about the partition type
where x and y refer to the locations returned by the find command
restart and then GRUB should be there.
Source:- In a linux book