Windows - Photoshop CS4-CS6
You use Photoshop CS4 or newer up to CS6 on Windows.
64 bit or 32 bit?
Photoshop comes in two flavours, 64 bit and 32 bit. If you use a 32bit version of Windows, you only have the 32bit version, choose 32 bit below.
If you use a 64 bit version of Windows, you probably have both 64 bit and 32 bit versions of Photoshop. Which one do you want to use?