It has to do with your operating system version. If your Windows is 64-bit, then you need 64-bit Autodesk products and vice-versa for 32-bit. You can find out by the system properties of your Windows. You can get there many ways. Right-click My Computer and go to Properties, or click System from Control Panel, or hold the Windows keyboard key down and hit the Pause/Break key at the same time. In any case, on the general tab, it should tell you your version of Windows.
Also, if you have a Program Files (x86) fiolder on your C:\ drive, you have 64-bit Windows. You cannot install 32-bit on 64-bit or vice-versa, so be sure you get the correct version before installing. If you get the error that this program will not install on your version of Windows, then you have the incorrect version.