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  If you've installed Windows 7 Beta on your PC, you'll need to back up your data, and do a clean installation of the RC. Then you'll need to reinstall your programs and restore the files, settings, and other information you want to use for testing.
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  While we consider this a stable and high-quality pre-release version of Windows, the RC is not the finished product. It could crash your computer or cause you to lose important files or information.
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  You're pretty much on your own as you test the software. You need to be comfortable setting up and troubleshooting a PC. If you have a problem, you can find a lot of help from other testers online, but we're not providing formal tech support.

Things you should have before starting:

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  Dedicated test PC: Please don't test the RC on your primary home or business PC.
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  Data backup: The information on your PC will not be saved during installation. Back up your PC before you start, and frequently during testing. You can back up files to an external hard disk, a DVD or CD, or a network folder.  If you're running Windows 7 Beta on the PC you'll use for test, you can use Windows Easy Transfer to create a backup on an external drive. To find and launch Windows Easy Transfer, just type "easy" in the Start search box.
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  Recovery disc: A recovery disc restores your computer to its out-of-the-box state. Most computers come with one. If yours didn't, contact your PC maker.
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  Antivirus program: Make sure your antivirus program is up-to-date, run it, then disable it before installing Windows 7 RC. When you finish the installation, remember to re-enable the antivirus program.
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  Internet connection: Be sure to connect to the Internet after you finish the installation. Being connected helps make sure you get useful security and hardware updates. We recommend you keep your PC set to automatically check for updates through Windows Update.

Or visit this link to know more...

http://www.microsoft.com/windows/windows-7/get/installation-instructions.aspx