Hi Nithin,
Most people have faced this problem. The issue here is that when you
installed windows for the second time it wiped out your
bootloader( may be GRUB ) and installed it's own ( without asking
you ).
Now you just have to reinstall the bootloader that can boot both linux
& windows such as GRUB.
You can boot into a live CD of your distro and go to the terminal and
do,
sudo grub
// GRUB prompt comes
> find /boot/grub/stage1
// gives 1 or more partitions where grub is installed
// in the format (hdx,y)
// x -> hdd number
// y -> partition number
> root(hdx,y)
> setup(hdx)
> quit
now reboot tho see the GRUB menu.