Fix:
It is always desired that you install windows before you install Linux as the installation other way round would result in corrupted boot-loader and you will not get the option to boot from Linux.

Due to wrong sequence of installation of windows over a Linux system, windows erases the existing boot-loader from boot sector of the hard disk. It overwrites existing boot sector with the windows boot-loader (NTLDR).

But don't worry as you can simply fix this using a Linux bootable CD /DVD.

Boot the computer with the Linux bootable CD / DVD.

1. At the boot prompt, type 'linux rescue' (without quotes) and hit enter.
2. Continue with the default options through the prompts.
3. When you get to a bash prompt, type 'chroot /mnt/sysimage' (I think that's the path).
4. Then, type '/sbin/grub-install /dev/hda' (assuming, hda is your system drive).

5. Eject the cd /dvd and reboot your computer ,you will see the default grub bootloader which allows you to boot both with windows and Linux

Hope this info fixes up the trouble.