Are you transferring it over to the new PC? If so, do you have an OEM version of Windows 7? If you don't, you will have to deactivate Windows 7 on your old PC before installing it on a new one. If you do have an OEM copy, you will have to purchase a new copy of Windows 7, as the OEM version of Windows 7 is tied to the first motherboard it was installed on.
Downloading and installing: Once you download Windows 7 from digitalriver, use
Windows 7 USB/DVD Download Tool to mount the .ISO file on your flash drive or disc. Put the DVD in the optical drive or plug the flash drive into a USB port, then unplug all USB devices from your PC except mouse and keyboard and boot up your computer. Spam the delete key as soon as you press the power button. You should now be in your mobo's BIOS. On the right side you should see a "Boot Priority" list. Move the HDD down and select the DVD/USB as the first box. Exit the BIOS and the computer should boot up to the Windows 7 installation screen. Just follow the steps, and the last step should be activating Windows with your product key.
Disk install: If you have the disk that came with your copy of Windows 7, just pop in the disk that came with your copy of Windows 7 (it should automatically boot from it since there is nothing else to boot from) and follow the instructions on-screen. Same as last time, just follow the steps, and the last step should be activating Windows with your product key.