Today’s PCs come in two varieties: those running a 32-bit operating system and those running a 64-bit operating system. The difference is the amount of information the PC’s processor can handle at any precise moment. While a computer designed for a 64-bit operating system has vastly more potential computing power, it requires fundamental changes to the way its software is designed.

The word "bit" refers to the way computers deal with information in binary, where all data is listed as a string of digits which can either be a 0 or a 1. Each of these digits is known as one bit, meaning a 32-bit processor can process 32 digits at once. Don’t confuse this with memory, which measures the total amount of information a computer can remember without needing to use a storage device such as a disk. While the figure for memory will be much bigger, a computer can’t process all of that information at once.

The software for a computer with a 32-bit processor, including operating systems such as Windows, has to be specifically written to match that processor. The same applies to 64-bit processors. Microsoft produced 32-bit and 64-bit editions of Windows XP and Vista, and will do the same for Windows 7.

There are also some significant mathematical limits to the two different types of processor. A 32-bit processor can only work with a maximum of 4GB of memory, and this is usually limited to 2GB for any one program. A 64-bit processor could theoretically work with 17 million GB of memory. A 64-bit processor can also carry out some tasks twice as quickly.

The memory limitations of a 32-bit processor started to become clear with Vista, which uses a large amount of memory and can make it difficult to run multiple programs at once without using up even a full 4GB of memory. Meanwhile, it once seemed impossible that any one program would need more than 2GB of memory, but some modern video games have hit that limit. For these reasons, 64-bit processors will likely start to become much more popular, which will increase the number of consumers interested in buying a 64-bit operating system.

The 64-bit editions of Windows can run some software designed for the 32-bit edition through a special compatibility mode, but the results can be very varied. Using a 64-bit operating system can also cause problems with drivers. These are small pieces of software that coordinate each hardware device with an operating system such as Windows.