http://www.pcworld.com/article/248995/how_to_install_windows_7_without_the_disc.html
Many computers ship without Windows 7 installation discs. Instead they come with "recovery" discs and/or partitions that claim to restore your PC to its "factory fresh" state, but include a ton of bloatware (free trials, media players, useless utilities, and the like) along with Windows. Removing each piece of bloat is time-consuming, and just not the same as truly starting fresh.
In this article, you'll learn how to make your own Windows 7 installation disc (or USB flash drive) and use the valid product key you already have to perform a fresh reinstallation. The following method is effective and legal, and doesn’t require a lot of technical expertise. (If you do need help with bloatware, however, read "What to Do With a New PC.")
However, we don't recommend that you delete your recovery partition or throw away your existing recovery discs. Although many people have had success with the following procedure, others have not. Some users have not been able to activate their copy of Windows even after entering a valid product key, while others own PCs that require software not included in a Windows 7 ISO. You have no way to know whether this procedure will work for you until you try, and you need a backup plan in case it doesn’t.
Also, save your files and drivers to external media before you start. Reinstalling Windows wipes out all personal data and causes a loss of functionality in devices for which Windows lacks an adequate built-in driver. Plan on loading the latest device drivers from media you create before reinstalling Windows.
Reinstalling Windows 7, Step by Step
1. Find your Windows 7 product key: Typically this 25-character alphanumeric string is printed on a sticker affixed to your PC or on documentation included with your PC. Alternatively, you can use a keyfinder program such as Magical Jelly Bean Keyfinder to pull your product key from the Registry. You need your product key to reinstall Windows.
2. Download the ISO file for the version of Windows 7 you own:
The above links point to Digital River, a licensed distributor of Microsoft software. These downloads are each over 3GB, and your product key will work only with your edition of Windows, so make sure to select the right one.
3. Download the Windows 7 USB/DVD download tool. This utility lets you copy your Windows 7 ISO file to a DVD or USB flash drive. Whether you choose DVD or USB makes no difference; just confirm that your PC can boot to the media type you select.
4. Boot your computer using the Windows 7 DVD or USB flash drive. Follow the prompts to prepare the destination hard drive and install Windows.