It's generally easier to install Windows first, and Linux afterwards (as Linux plays nice with Windows, but Windows doesn't play nice with Linux). Assuming you want to retain your current Linux Mint installation, you'd have to:
- Boot from the Linux Mint installation DVD/USB, and use the GParted application to resize your current Linux Mint partitions to make room for Windows. Then create an empty primary partition for Windows (NTFS), so the Windows installer will offer to install there.
- Install Windows to the prepared primary partition. The installation steps are not that long or complicated, see here for example screenshots from the installer: http://stuff.seans.com/2008/11/07/windo ... reenshots/. The hunt then starts in Windows as you will have to find and manually install drivers for your hardware devices and peripherals. Some may have come on the driver DVDs with your hardware devices or peripherals, others you will have to find online. Perhaps these days Windows at least helps you out with that; my last Windows install was XP and at that time you had to do it all yourself. If you have older hardware devices or peripherals, prepare to be disappointed as manufacturers don't necessarily provide drivers for their older products on Windows 7.
- Finally boot from the Linux Mint installation DVD/USB again and follow the steps here to reinstall GRUB to the MBR (/dev/sda): https://help.ubuntu.com/community/Boot- ... _in_Ubuntu. You need to do this because Windows will have overwritten the MBR, installing its own bootloader and thus overwriting the Linux bootloader (GRUB). The Linux bootloader will allow you to boot either Linux or Windows; you get a menu during boot to choose which to boot.