I think you can download it but for its trial version only., if you really want to experienced the real AP CS2 try to bought it.,
If you're a student you can get a really cheap version of it (about $200) but you're not allowed to use it for commercial purposes. Hope that helps.
You can use the Gimpshop addon too if you are used to Photoshop key bindings and screen layout. It changes the Gimp interface to behave more like Photoshop. Also, Pixel32 is a Photoshop clone that is available for much cheaper than Photoshop. It runs on Windows, Linux and OS X.