Unlike Apple, Microsoft does not control the hardware that its software runs on. This means that Apple can more easily move all its users to an x64 operating system: all Macs currently have 64-bit CPUs, and Snow Leopard is rumored to be a 64-bit-only release. Windows 7, on the other hand, will still be released in x86 and x64. Microsoft would prefer not to make Windows 7 available on computers with 32-bit CPUs (indeed, Windows 7 Server will be x64-only), but the decision is driven by software compatibility demands.
Many businesses still use 16-bit applications, which cannot run on a 64-bit operating system, or have 32-bit applications that for one reason or another don't run properly in an x64 environment. Few software developers offer x64 programs, and Microsoft doesn't want to hurry them up; the software giant wants the industry to make the move by itself. Apparently, this has already started. On the Windows Vista Team Blog, Microsoft has posted details of how the PC industry is moving from 32-bit to 64-bit PCs:
We've been tracking the change by looking at the percentage of 64bit PCs connecting to Windows Update, and have seen a dramatic increase in recent months. The installed base of 64bit Windows Vista PCs, as a percentage of all Windows Vista systems, has more than tripled in the US in the last three months, while worldwide adoption has more than doubled during the same period. Another view shows that 20 percent of new Windows Vista PCs in the US connecting to Windows Update in June were 64bit PCs, up from just 3 percent in March. Put more simply, usage of 64bit Windows Vista is growing much more rapidly than 32bit.
This rapid growth may appear to have come from nowhere, but on closer inspection, it hasn’t. The major advantages of running a 64-bit installation of Vista is the ability to use 4GB or more of RAM and to use 64-bit applications. Although 32-bit operating systems can use more than 4GB of memory, for compatibility reasons MS limits 32-bit desktop OSes to 4GB. Prices for RAM have fallen, however, and OEMs are offering PCs with 4GB of RAM and more, forcing the switch to 64-bit Windows. The realization that Vista x64 has significantly superior compatibility to XP x64 is also starting to sink in. In Vista, 64-bit drivers are required for WHQL certification, and so many hardware manufacturers that were previously ignoring x64 have finally started to release x64 drivers.
Microsoft is also playing its part in the move to 64-bit. In addition to its 64-bit ready webpage, the company recently launched the Windows Vista Compatibility Center into beta phase, which will tell users whether a given product is 64-bit-compatible or not. x64 is clearly the future. If Redmond does indeed follow through with its decision to offer x86 and x64 versions of Windows 7 (and there's no indication that it won't), the company should at least make the effort to get OEMs to offer x64 by default on systems that can run it.
Further readingWindows Vista Team Blog: Windows Vista 64-bit Today