xp+ubuntu 12.04双系统，重装xp为win7，装好win7后恢复ubuntu的启动项。
《百度一下，你就知道》http://hi.baidu.com/shuye216/blog/item/2694891e3ef6ae094134173f.html
应用LiveCD启动光盘选择“试用而不安装”，进入后打开终端：
1、sudo fdisk -l
找到 id=83 选项，获取ubuntu的分区。
Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x91c6943c
Device Boot Start End Blocks Id System
/dev/sda1 * 63 102414374 51207156 7 HPFS/NTFS/exFAT
Partition 1 does not start on physical sector boundary.
/dev/sda2 102414436 597938175 247761870 f W95 Ext'd (LBA)
Partition 2 does not start on physical sector boundary.
/dev/sda3 597938176 776771583 89416704 83 Linux
/dev/sda4 776774880 976768064 99996592+ 83 Linux
/dev/sda5 102414438 348176744 122881153+ 7 HPFS/NTFS/exFAT
Partition 5 does not start on physical sector boundary.
/dev/sda6 348176808 593939114 122881153+ 7 HPFS/NTFS/exFAT
/dev/sda7 593940480 597938175 1998848 82 Linux swap / Solaris
2、sudo -i
3、mount /dev/sda2 /mnt
4、mount /dev/sda2 /mnt/boot 这一步是有给 /boot 单独分区的命令，若没单独分区跳过这步
5、grub-install --root-directory=/mnt /dev/sda
6、重启