You have to make a bootable disc with your windows 7 iso file.
I would suggest you to use PowerISO, or even Nero.
When you do that, you should insert the disc in your drive, restart windows and normally, BIOS will tell you to press any key to boot from CD/DVD. If you don't get that message, press delete or the f-button that your system uses for booting. :)
tip : Windows 7 can be installed from your previous windows interface too.