There are a few ways you can do it.
here are a couple of user friendly ways.
1)Use the LiveDVD/CD run through the install gui (don't reformat anything) until you get to the grub install section and use it to reinstall grub and then quit the installer.
2) Use supergrubdisk --> http://www.supergrubdisk.org/
3) slightly more complicated -->
3a. Pop in the Live CD, boot from it until you reach the desktop.
3b. Open a terminal window or switch to a tty.
3c. Type "grub"
3d. Type "find /boot/grub/stage1". You'll get a response like "(hd0)" or in my case "(hd0,2)". Use whatever your computer spits out for the following line
3e. Type "root (hd0,2)", or whatever your harddisk + boot partition numbers are (my /boot is at /dev/sda7, which translates to hd0,6 for grub).
3f. Type "setup (hd0)", ot whatever your harddisk nr is.
3g. Quit grub by typing "quit".
If you check the wiki, use the search function and google you can find the answer to most of your questions.
Remember to mark your thread [Solved] if your issue is fixed.