Windows never recognises linux installations; it just overwrites the boot on the disk.
You need to go back to the linux install disk, restart using that to boot, and then re-write the grub system;
start a terminal, & do this:
mount /dev/sda /mnt
mount --bind /proc proc
mount --bind /sys sys
mount --bind /dev dev
mount --bind /usr usr
It is possible to shorten this but may then not always work; what you are doing is mounting your main drive (I've assumed one drive on the PC) and lying to the system, in effect, to make the grub programs run on your drive not the boot disk, then re-install the bootloader; grub will recognise Windows, and Linux, fix things, and then you reboot (removing the CD or USB you booted from) - et voila.
This has been covered a number of times before, you're not alone!