After installing windows->Linux at your windows is corrupt than you may install windows again to your previous partition after installation of windows 2nd time we will not get/see the lebel of linux though yours linux partition are okey to overcome above problm to the flowing step by step->
1. boot from bootable cd/dvd of linux. (same version)
2. boot: linux rescue (press enter)
3. type next/ok/continue expect only network is no.
4. #chroot /mnt/sysimage. (press enter)
5.#fdisk -l you wiill see your hard drive is hda/sda.
6.#grub -install /dev/hda or /dev/sda
7.#reboot
Hopefully your pc (windows+linux) will be okay.