Download the reason installer from the account section of the propellerhead website. (If you have the original CD then you can skip downloading and adjust these instructions appropriately). The installation files should be in a .zip file, you will need to extract these to a folder somewhere (right-click on the .zip file and select "Extract All...").
Its possible to have both the 64-bit and 32-bit versions of reason installed simultaneously. Make sure that any shortcut you use to start Reason is starting the correct version - in Reason 6.5.3, both the 32-bit and 64-bit shortcuts in the start menu are given the name Reason, so its hard to distinguish between the two! You can check the shortcut you are using launches the correct version of Reason by verifying the installed directory - 32-bit Reason will install into a directory within "C:\Program Files (x86)\", while the 64-bit Reason will instal into "C:\Program Files\". 2b1af7f3a8