You problem is quite logical: You installed linux, and after that you installed windows. Windows has overwritten the masterboot record, so takes control of the boot-process, ignoring linux (windows is used to being the only OS on a computer apparently).
The solution: start linux with the boot-floppy you have made during install (I hope???). After booting into linux, install grub in a shell as root probably with the command
but be sure to check the man-page of grub-install
Next time you boot your computer it should boot with grub. If windows isn't in the bootlist, edit /boot/grub/menu.lst or /boot/grub/grub.conf (isn't always the same file). In the configuration file is mostly explained how to boot windows with grub.