Before You Begin: You will want to make sure that all of your data from your old computer or hard drive has been saved to an external location. If you want it on your new system, then go ahead and save it! You'll be doing a clean install of Windows, so expect to spend a good deal of time downloading drivers and software again. If you have an installation disk for drivers (usually bundled with your system instead of an operating system disk) this is the best method, though you might need to download drivers before-hand and save them on a backup disk or external drive. Finally, and most importantly, you will need the activation key for your copy of Windows. You can usually find this on a sticker attached to your laptop or desktop. If not, use a key-finder program which will provide you with the activation key for your copy of Windows.
Step One: Find an ISO file of your copy of Windows. Navigate to your Computer Properties and copy down your OS, the service pack number, and the bit-type (32 bit, 64 bit computing, etc.). Taking this information, perform an internet search for an ISO file of the same specifications. These are files which are a single, uncompressed operating system file which are used for installs. Expect this file size to be a little over three gigabytes. Download it to the desktop of your computer.
Step Two: Download the Windows 7 USB / DVD tool. Send this file to your desktop as well, and following the download, go ahead and install it on your computer.
Step Three: Ready your media. Whether you're going to use a flash drive or a DVD to write the ISO file is up to you. Insert it into your computer at this time, ensuring that either media is completely blank. Run the Windows 7 USB / DVD tool. This will prompt you to select your ISO file. Do so, then select your external media file. Follow the directions to copy the ISO file to your USB drive.
Step Four: Boot up your new computer, or computer with the new blank hard drive. No operating systems should be detected. Plug in your flash drive or insert your DVD with the ISO file on it. Enter setup (I hit F2 from boot on my HP DC7600 , though some computers will offer F11 or F12 for setup options) and choose that as the boot media. Following this, you should boot into a Windows 7 fresh installation! Set up your account, install your drivers and programs, and you're good to go!