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 may have both 64 bit and 32 bit versions of Photoshop. You can check if your Photoshop is 64-bit or 32-bit in the System Info. From the Photoshop Help menu, choose "System Info...". The top-most line shows the Photoshop version. If it has x64 at the end, you are using 64-bit and then you also need 64-bit plug-ins. If this line ends in x32 you are using 32-bit Photoshop.