Look here: 32-bit and 64-bit Windows: frequently asked questions
As stated by ignatzatsonic, backup your data first to an external drive, and then clean install Windows 7 64-bit.
Make sure you have any drivers
you need and the installation discs for your applications.
Do you have the retail version of Windows? If so, you should have both the 32-bit and the 64-bit disc.
When you do go to the 64-bit version you'll notice two Programs directories:
- Program Files, which is the container for any 64-bit programs.
- Program Files (x86), which is the container for any 32-bit programs.