http://windows.microsoft.com/en-us/windows/installing-reinstalling-windows#1TC=windows-7
''Before you begin, make sure that your computer can run a 64-bit version of Windows. For more information, see 32-bit and 64-bit Windows: frequently asked questions.
The Custom option installs a new copy of Windows 7 on the partition you choose. This erases your files, programs, and settings.
Back up any files and settings you want to keep so that you can restore them after the installation is complete. You'll need to reinstall your programs, so make sure you have the installation discs and product keys for the programs you want to use in Windows 7, or setup files for any programs you downloaded from the Internet.''