I have no experience with Mint, but I suspect that the following error occured. During installation, you were given the option of installing grub to the MBR (master boot record) of the hard drive, or to the boot sector of the Mint partition. If the default was to install to the boot sector of the partiton, or if you selected that option, that would explain your problem.
Insert the Mint install disk and power up. Does the Mint install disk offer the option to boot from hard drive? If so, select that option. Once in Mint, you can run grup-install (hd0) to install grub to the MBR (do some research to verify the command I just gave you).
Note that you may have to manually edit the grub2 configuration files to add windows to the boot menu.
Or, you could re-install Mint. When the installation reaches the grub install, be careful to select installing grub to the MBR.