Repair Windows 7 mbr

I have a laptop, my hard drive into two partitions, one with Windows 7, the other with Ubuntu.Yesterday I formatted my Ubuntu partition (which really helped me more) to save XP Instead compatibility issue. Except that my XP cd is too scratched, he has planted the setup, but the rub is that he has screwed up my Mbr ...So I boot the liveCD ubuntu, have re-installed, history of having an OS and a