I installed Linux OS on the PC. After that, I installed Windows 7. Now, I am unable to load Linux again. How to install Grub in Windows 7?
You'll have to boot to an Ubuntu live CD to reinstall grub.
From the tutorial located here:
Mount the partition your Ubuntu Installation is on. If you are not sure which it is, launch GParted (included in the Live CD) and find out. It is usually a EXT4 Partition. Replace the XY with the drive letter, and partition number, for example:
sudo mount /dev/sda1 /mnt.
sudo mount /dev/sdXY /mnt
Now bind the directories that grub needs access to to detect other operating systems, like so.
sudo mount --bind /dev /mnt/dev && sudo mount --bind /dev/pts /mnt/dev/pts && sudo mount --bind /proc /mnt/proc && sudo mount --bind /sys /mnt/sys
Now we jump into that using chroot.
sudo chroot /mnt
Now install, check, and update grub.
This time you only need to add the drive letter (usually a) to replace X, for example: grub-install /dev/sda, grub-install –recheck /dev/sda.
grub-install /dev/sdX grub-install --recheck /dev/sdX update-grub
Now grub is back, all that is left is to exit the chrooted system and unmount everything.
exit && sudo umount /mnt/dev && sudo umount /mnt/dev/pts && sudo umount /mnt/proc && sudo umount /mnt/sys && sudo umount /mnt
Shut down and turn your computer back on, and you will be met with the default Grub2 screen.
The easiest way to ensure a dual booted machine is to load Windows first, and then load Linux.
This ensures that GRUB will see all the drives and mount points. On first boot, GRUB will load, and then you can then arrow down and choose which OS to boot. Your Windows boot drive will be displayed last in the listing, behind Linux.
Even better, to avoid rebooting your machine to get from one OS to the other, load Windows, download VMware Player (it's free), and then load Linux as a virtual machine (the other way around, is OK too, however the Linux version of VMplayer is a little tricky, so you might want to use VirtualBox).