Quick way to check whether your flavor of Windows is 64-bit:
Go to "start"
On older versions of Windows, click "Run..." ... or on newer flavors, such as Windows 7, you can just type in the search box
Type "dxdiag" (without the quotes)
Under "System Information," the third line will tell you what version of Windows you have, and should say "64-bit" if your version of Windows is 64-bit.