Windows Vista comes in 32-bit and 64-bit editions depending on which CPU type you have. The 64-bit version will only work on 64-bit processors, however the 32-bit version can run on all types.
Some users will have a processor capable of running the 64-bit version of Vista but have the 32-bit version installed by default. This won't cause any problems, but it does mean you may miss out on some advantages (such as being able to utilise large amounts of memory). If you are in this situation, you can check to see if your computer can handle the 64-bit version of Windows Vista using the following steps.
First, open the Control Panel by clicking on the Start Menu > Control Panel: