I am assuming that you wish to do dual boot installation of RHEL with Windows 7. If you have plans to run RHEL on virtual machine such as Virtualbox then instructions differ.
1 : You need one spare partition in order to install Linux. So make sure that your drive is free of data where you wish to install the linux. If your computer has single drive then you may have to resize the existing partition. You can use the disk management utility from within the windows to resize the windows existing partition. This way you can use RHEL on that second partition. Open disk management utility and resize your partition.
2. BIOS : Boot into your computer BIOS. And set it to read DVD ROM. You can also set it for the USB port if you have made RHEL USB drive. In such case you just have to switch the option for the USB port.
3. Install disc: Make sure you have all the necessary CDs or DVD for the RHEL.
4. Installation : Insert the disc into the drive or USB device into port and boot into the installation menu.
5. Partition : Once the menu asks you to partition the specific drive. Go ahead and specifically point the drive for the installation.
6. Follow the installation instruction for making swap partition and other necessary things for RHEL.
7. Dual boot options: RHEL does detect your dual boot option so you have to specify the dual boot settings. Once you do this it starts the installation process where you have to choose the package that you want to install. You can choose the standard packages or choose to customize the settings.
8. After you finish the installation, remove the disc and restart the system.
You'll be presented with the GRUB menu that gives you option to boot into Windows or RHEL. If by any chance if your GRUB menu gets corrupted then you can use either Windows disc or RHEL disc to repair the boot menu.