Whenever you reinstall Windows with an existing Linux installation already present on the hard drive, the MBR gets an overwrite. You can restore GRUB to the previous settings by booting a Linux Live CD and following the steps below:
1. Insert the Live CD and reboot your computer;
2. After reaching the desktop, open up a Terminal and do a su – to access the root account. In Ubuntu you’ll have to previously give a password to the root account with sudo passwd root;
3. Type grub in the root prompt;
4. The Grub prompt has appeared and you should type find /boot/grub/stage1 in it then hit Enter. You’ll get an output similar to (hd0) or (hd0,2);
5. Type root(hd0,x) if this is the output you got earlier then hit Enter;
6. Type setup and hit Enter;
7. Type quit and hit Enter;
8. Reboot the system and remove the Live CD from the tray.