Windows 7 is already a big hit for Microsoft, according to market-share tracker Net Applications, which shows it rising past all the extant versions of Linux and Windows except Vista and XP and into fourth place hot on the heels of the Mac OS X 10.5.

One of its most talked-about features is a version of XP built right in to some editions, so it can run in native mode on a virtual machine all those applications that never made the leap compatibility with Windows Vista.

Except XP Mode doesn't come automatically; you have to install it. And it doesn't come with all editions of Windows 7.

[ For timely virtualization news and expert advice on strategy, see CIO.com's Virtualization Drilldown section. ]

Users running Professional, Ultimate or Enterprise have to download both XP Mode and Virtual PC, on which it runs. Those with Home Premium or Starter are stuck; Virtual PC not only doesn't come with those editions, Microsoft theoretically doesn't allow Virtual PC to even run on anything but Vista, XP or the three more exalted editions of Windows 7.

That's not to say Virtual PC doesn't run there, anyway, however. And, fortunately, the installation procedure is the same for Virtual PC whether you're licensed for XP Mode or not.

[ For complete coverage on Microsoft's new Windows 7 operating system -- including hands-on reviews, video tutorials and advice on enterprise rollouts-- see CIO.com's Windows 7 Bible. ]

I loaded and ran it on a laptop running 64-bit Windows 7 Home Premium on an Intel Core 2 Duo with 4GB of memory. Here's how to get going:

Step 1: Check your Processor

Intel and AMD have both built hooks into their processors that allow the host and guest operating system (the virtual machine) to trade off tasks more smoothly. Virtual PC will work on chips that don't have those hooks, but not well. Microsoft provides a free utility to check your processor. Intel and AMD have their own utilities as well, if you want to double check. Intel Processor Identification Utility; AMD Virtualization and Hyper-V compatibility Check.

Once you know if the silicon supports it, check to see if your BIOS is set up to use those hooks. Chances are, for most desktops and laptops, it's not. Microsoft offers instructions and links to specific manufacturers here.

Step 2: Download Virtual PC

Microsoft requirements call for a 400 MHz or above Pentium-compatible processor, 35 MB of disk space and Windows XP or Vista. There are 32-bit and 64-bit versions; Virtual PC cares about the difference. The newest version of Virtual PC supports USB peripherals and are supposed to be able to support 64-bit operating systems within the VM as well. Either way, get the right edition for your machine.