For most would-be Windows 7 users, a 64-bit version of Windows 7 is the right move. But if you don't have sufficient RAM (at least 4GB), or you rely on devices that don't have supporting 64-bit drivers, or you need to upgrade an existing 32-bit installation, 32-bit Windows 7 might be the better choice. A few editions of 64-bit Windows 7 provide a Windows XP Mode that solves some backward-compatibility problems, but it isn't a universal panacea. In speedy Q&A format, here's just what you need to know.
Q. Is my PC supported under 64-bit Windows 7?
A. Most PCs manufactured in the past three years (i.e. after Vista's debut) are capable of running 64-bit Windows 7. The exceptions are those that ship with low-end CPUs that don't support the AMD or Intel 64-bit extensions. Examples include Intel's Atom line of low-powered CPUs and early Intel Core CPUs, like the Core Duo (not Core 2 Duo). If you're not sure what kind of CPU is in your system, or whether the CPU supports 64-bit operation, you can use the free Intel Processor Identification Utility to find out.
Q. What about all of my peripherals?
A. As with PC support, most peripherals manufactured in the Vista era work with 64-bit Windows 7. However, legacy hardware support is a hit-or-miss proposition. Some manufacturers -- for example, vendors of network interface cards or disk storage controllers -- are more up to date than others, thanks in part to the fact that they've been supporting 64-bit computing under Windows Server since the 2002-2003 timeframe. Devices that don't have obvious ties to the datacenter (such as custom input devices, multimedia hardware, and some printers) can be harder to integrate since they were manufactured at a time when 64-bit desktops were a rarity. Your best bet is to do a Web search to see if the manufacturer has published a 64-bit device driver and, barring that, if other users have come up with a solution.
Q. Can I use a 32-bit device driver under 64-bit Windows 7?
A. No. A device driver is privileged code that runs in the same address space as the Windows kernel. As such, it needs to match the architecture of the kernel itself. Some manufacturers bundle both 32-bit and 64-bit drivers within a single installation package, leading casual observers to sometimes misreport that a 32-bit driver worked under 64-bit Windows. However, while 32-bit drivers are not directly supported in 64-bit Windows 7, 64-bit Windows 7 users can install 32-bit drivers in Windows XP Mode and use USB-based printers and other USB-based legacy devices with the Windows XP virtual machine.
Q. Can I use Windows XP Mode with 64-bit Windows 7?
A. Yes. Windows XP Mode is fully supported under 64-bit Windows 7. In fact, using a Virtual Machine Monitor (VMM), like the Windows Virtual PC 7 product that underlies Windows XP Mode, is one of the only ways to use a 32-bit device driver under 64-bit Windows. The lone caveat is that the device must use a USB interface; legacy hardware that uses a proprietary expansion card or dongle will likely not work with a VMM solution like Windows XP Mode.
Q. What exactly is Windows XP Mode, and how do I get it?
A. The simple answer is that Windows XP Mode is a virtual machine containing Windows XP SP3 that runs under Windows Virtual PC 7. It is available as a free download to users of Windows 7 Professional, Ultimate, and Enterprise editions.
The more precise answer is that Windows XP Mode is a native 64-bit application (actually, a series of 64-bit services and device drivers) that creates a separate, native 64-bit process emulating a 32-bit PC environment.
It's important to note that Windows XP Mode is limited to creating a 32-bit virtual environment. This is true even though the underlying Windows Virtual PC software is itself 64-bit and running on the x64 version of Windows 7. So, while you can install 32-bit Windows XP (or 32-bit Vista or 32-bit Windows 7) as guests inside the Windows Virtual PC environment (which is, after all, a generic VM solution with some additional integration for the Windows XP Mode image), you cannot install 64-bit Windows XP or the x64 versions of Vista or Windows 7.